Pirate’s surprised expressions on air (7)
I believe the answer is:
corsair
'pirate's' is the definition.
(corsair is a kind of pirate)
'surprised expressions on air' is the wordplay.
'surprised expressions' becomes 'cors' ('cor' is an expression of surprise).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'cors'+'air'='CORSAIR'
(Other definitions for corsair that I've seen before include "A swift pirate ship" , "Pirate (archaic)" , "Privateer" , "work by Verdi" , "20 across" .)