Weakling, politician joining women's group (4)
I believe the answer is:
wimp
'weakling' is the definition.
(wimp is a kind of weakling)
'politician joining women's group' is the wordplay.
'politician' becomes 'mp' (Member of Parliament).
'joining' means one lot of letters go next to another.
'women's group' becomes 'wi' (Women's Institute).
'mp' after 'wi' is 'WIMP'.
(Other definitions for wimp that I've seen before include "Feeble or ineffective man" , "Feeble individual" , "A weak ineffectual man" , "Softie" , "Softy" .)