About to apply reason (5)
I believe the answer is:
cause
'reason' is the definition.
(I've seen this before)
'about to apply' is the wordplay.
'about to' becomes 'ca' (I've seen this in other clues).
'apply' becomes 'use' ('use' can be a synonym of 'apply'**).
'ca'+'use'='CAUSE'
(Other definitions for cause that I've seen before include "Grounds" , "A movement or charity- usually good" , "why some people fight" , "Bring about" , "Charity - motive" .)