One in classic group of three (5)
I believe the answer is:
triad
'group of three' is the definition.
(I've seen this before)
'one in classic' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'classic' becomes 'trad' (abbreviation of traditional).
'i' placed within 'trad' is 'TRIAD'.
(Other definitions for triad that I've seen before include "three making a sound?" , "Chinese secret society" , "Set of three; chord" , "Chinese criminal society" , "Criminals" .)