A political group mostly in pieces (5)
I believe the answer is:
apart
'in pieces' is the definition.
(I've seen this before)
'a political group mostly' is the wordplay.
'political group' becomes 'party' (a political party).
'mostly' means to remove the last letter (most but not all of the word).
'party' with its final letter taken away is 'part'.
'a'+'part'='APART'
(Other definitions for apart that I've seen before include "Divided" , "Unconnected" , "Separate in a trap" , "set aside" , "Independently" .)