Supporter missing start of political gathering (4)
I believe the answer is:
ally
'supporter' is the definition.
(an ally is a friend or supporter)
'missing start of political gathering' is the wordplay.
'missing start of' means to remove the first letter.
'political gathering' becomes 'rally' (I've seen this in another clue).
'rally' with its first letter taken away is 'ALLY'.
(Other definitions for ally that I've seen before include "Team up" , "often helpful when cluing adverbs" , "A friendly and reliable country" , "Friendly nation" , "Colleague, confederate" .)