Experts in aerospace science (4)
I believe the answer is:
aces
'experts' is the definition.
(ace is a kind of expert)
'in aerospace science' is the wordplay.
'in' indicates a hidden word.
'ACES' is hidden within 'aerospace science'.
(Other definitions for aces that I've seen before include "Top cards or people" , "never come back" , "Playing cards with single pips on their faces" , "holes in one" , "Champions" .)