Loud friend turns up -- panic! (4)
I believe the answer is:
flap
'panic' is the definition.
(thesaurus)
'loud friend turns up' is the wordplay.
'loud' becomes 'f' (A musical term).
'friend' becomes 'pal' (pal is a kind of friend).
'turns up' says the letters should be written backwards.
'pal' in reverse letter order is 'lap'.
'f'+'lap'='FLAP'
(Other definitions for flap that I've seen before include "Tab - tizzy - move wings" , "Flutter" , "Move wings - panic!" , "Use wings; be agitated" , "Minor panic; tab" .)