Women’s group taking on politician, a weak fellow (4)
I believe the answer is:
wimp
'a weak fellow' is the definition.
'women's group taking on politician' is the wordplay.
'women's group' becomes 'wi' (Women's Institute).
'taking on' is a charade indicator (letters next to each other).
'politician' becomes 'mp'.
'wi'+'mp'='WIMP'
(Other definitions for wimp that I've seen before include "Informally, an ineffectual man" , "Namby-pamby" , "Softie" , "Feeble or ineffective person" , "Feeble individual" .)