He's in aerodynamics (3)
I believe the answer is:
rod
'he's' is the definition.
'in aerodynamics' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ROD' is hidden in the letters of '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" .)