Sometimes supporter follows cause (12)
I believe the answer is:
occasionally
'sometimes' is the definition.
(I know that sometimes can be written as occasionally)
'supporter follows cause' is the wordplay.
'supporter' becomes 'ally' (ally can mean a supporter or friend).
'follows' means one lot of letters go next to another.
'cause' becomes 'occasion' (occasion can mean a reason or cause).
'ally' after 'occasion' is 'OCCASIONALLY'.
(Other definitions for occasionally that I've seen before include "From time to time" , "every now and then" , "Sometimes" , "At times" .)