He's in aerodynamics (3)
I believe the answer is:
rod
'he's' is the definition.
'in aerodynamics' is the wordplay.
'in' indicates a hidden word.
'ROD' can be found hidden inside 'aerodynamics'.
(Other definitions for rod that I've seen before include "Fishing pole" , "The fly fisherman's stick" , "Stick, symbol of punishment" , "Spare it and spoil the child, they used to say" , "Cane - shaft" .)