President with alien instrument (7)
I believe the answer is:
trumpet
'instrument' is the definition.
(trumpet is a kind of instrument)
'president with alien' is the wordplay.
'president' becomes 'trump' (Donald Trump).
'with' is a charade indicator (letters next to each other).
'alien' becomes 'et' (extraterrestrial).
'trump'+'et'='TRUMPET'
(Other definitions for trumpet that I've seen before include "Daffodil corona" , "Head" , "Brass musical instrument" , "brassy type" , "Brass wind instrument" .)