Part of committee, Thatcherite set (5)
I believe the answer is:
teeth
'set' is the definition.
(I know that teeth is a type of set)
'part of committee thatcherite' is the wordplay.
'part of' indicates a hidden word.
'TEETH' is hidden within 'committee thatcherite'.
(Other definitions for teeth that I've seen before include "Incisors maybe" , "Saw or gearwheel items" , "One may be armed to them" , "Cog wheel projections" , "Those of wisdom" .)