Phraseological units may be defined as specific word-groups functioning as word-equivalents; they are equivalent to definite classes of words. The part-of-speech meaning of phraseological units is felt as belonging to the word-group as a whole irrespective of the part-of-speech meaning of component words. Comparing a free word-group, e.g. a long day and a phraseological unit, e.g. in the long run, we observe that in the free word-group the noun day and the adjective long preserve the part-of-speech meaning proper to these words taken in isolation. The whole group is viewed as composed of two independent units (A + N). In the phraseological unit in the long run the part-of-speech meaning belongs to the group as a single whole. In the long run is grammatically equivalent to single adverbs, e.g. finally, firstly, etc.
So we distinguish set-expressions that are nominal phrases, functioning like nouns,
e.g.:Jack-of-all-trades - мастер на все руки,
ways and means - способы, средства,
Baker's dosen - чертова дюжина,
a thorn in the flesh - бельмо на глазу,
skeleton in the cupboard - семейная тайна;
verbal phrases, functioning like verbs:
to take the bull by the horn - действовать решительно,
to know the ropes - знать все ходы и выходы,
to flog a dead horse - попусту тратить время,
to put a finger into every pie - соваться во все дела,
to talk through one's hat - пороть чепуху;
adjectival phrases, functioning like adjectives:
spick and span - как с иголочки,
(as)cool as a cucumber - хладнокровный,
(as)poor as a church mouse - очень бедный,
(as) good as gold - золотой (о ребенке);
adverbial phrases, functioning like adverbs;
in a trice — в мгновение ока,
at sixes and sevens — в беспорядке,
before you can say «Jack Robinson» - мгновенно,
by hook or by crook - не мытьем, так катаньем; любыми средствами;
prepositional and conjunctional phrases:
as long as;
as well as;
in spite of;
as soon as;
interj ectional phrases:
well, I never! – кто бы мог подумать (ну и ну!),
by George! - (удивление, сожаление),
like hell! - черта с два,
my foot - держи карман шире,
my aunt! - боже мой! Здравствуйте, я ваша тетя!
my eye and Betty Martin! - вот так так!
So, phraseological units are included into the system of parts of speech.