Sounds like fantastic gun for acting president (6)
I believe the answer is:
reagan
'acting president' is the definition.
(US president and former actor Ronald Reagan)
'sounds like fantastic gun' is the wordplay.
'sounds like' indicates a 'sounds like' (homophone) clue.
'fantastic gun' becomes 'ray gun' (sci-fi weapon).
'raygun' sounds like 'REAGAN'.
'for' is the link.
(Other definitions for reagan that I've seen before include "40th US president, Ronald" , "Ronald . . . . . ., a film star and president" , "Ronald ..., ex-president of USA" , "political leader" , "head of state" .)