Reveal to Dicky what makes flight unnecessary (8)
I believe the answer is:
elevator
'what makes flight unnecessary' is the definition.
(I've seen this before)
'reveal to dicky' is the wordplay.
'dicky' indicates anagramming the letters (dicky can mean ill or non-functioning).
'reveal'+'to'='revealto'
'revealto' anagrammed gives 'ELEVATOR'.
(Other definitions for elevator that I've seen before include "An American lift in building" , "Gentle caress" , "may give you a lift" , "Farm herd" , "part of tail muscle" .)