Sisters gather round for parts of speech (5)
I believe the answer is:
nouns
'parts of speech' is the definition.
(I've seen this before)
'sisters gather round' is the wordplay.
'sisters' becomes 'nuns' (I've seen this before).
'gather' means one lot of letters goes inside another.
'round' becomes 'o' (round can mean a round object).
'nuns' enclosing 'o' is 'NOUNS'.
'for' acts as a link.
(Other definitions for nouns that I've seen before include "Words used as names of person, place or thing" , "Words that can be the subject or object of a verb" , "Words used as names of places or things" , "Words indicating names" , "Naming words" .)