They're instrumental in president meeting aliens (8)
I believe the answer is:
trumpets
'they're instrumental' is the definition.
The definition suggests an adverb but the answer is not.
'president meeting aliens' is the wordplay.
'president' becomes 'trump' (Donald Trump).
'meeting' is a charade indicator (letters next to each other).
'aliens' becomes 'ets' (ET is short for extraterrestrial).
'trump'+'ets'='TRUMPETS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for trumpets that I've seen before include "Proclaims loudly like elephant sounds" , "Brass musical instruments" , "Brass wind instruments" .)