Member in political party branch (4)
I believe the answer is:
limb
'branch' is the definition.
(I've seen this before)
'member in political party' is the wordplay.
'member' becomes 'm'.
'in' indicates putting letters inside.
'political party' becomes 'lib' (**).
'm' going within 'lib' is 'LIMB'.
(Other definitions for limb that I've seen before include "Large tree branch" , "Branch of a tree (risky to go out on)" , "Jointed appendage" , "as part of plane, say" , "Bough" .)