Reason about application (5)
I believe the answer is:
cause
'reason' is the definition.
(I know that reason can be written as cause)
'about application' is the wordplay.
'about' becomes 'ca' (short for circa).
'application' becomes 'use' (I've seen this before).
'ca'+'use'='CAUSE'
(Other definitions for cause that I've seen before include "A movement or charity- usually good" , "Give rise to" , "Grounds" , "why some people fight" , "Bring about" .)