As a flier, his heart's in the scheme (5)
I believe the answer is:
pilot
'as a flier his heart's in the scheme' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'his heart's in the scheme' is the wordplay.
'heart's' indicates the central letters (I've seen 'heart' mean this).
'in the' means one lot of letters goes inside another.
'scheme' becomes 'plot' (plot is a kind of scheme).
The central letter of 'his' is 'i'.
'i' put within 'plot' is 'PILOT'.
Can you help me to learn more?
(Other definitions for pilot that I've seen before include "Steer, test (scheme)" , "Navigator" , "Plane flyer" , "trial episode" , "Steer (a ship or plane)" .)