Captain supporting call for MC (10)
I believe the answer is:
ringmaster
'mc' is the definition.
(I've seen this before)
'captain supporting call' is the wordplay.
'captain' becomes 'master' ('master' can be a synonym of 'captain'**).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'call' becomes 'ring' ('ring' can be a synonym of 'call').
'master' after 'ring' is 'RINGMASTER'.
'for' acts as a link.
(Other definitions for ringmaster that I've seen before include "Circus controller" , "showman" , "Top top man" , "Circus leader" , "Circus compere" .)