About to employ reason (5)
I believe the answer is:
cause
'reason' is the definition.
(I've seen this before)
'about to employ' is the wordplay.
'about to' becomes 'ca' (I've seen this in other clues).
'employ' becomes 'use' ('use' can be a synonym of 'employ'**).
'ca'+'use'='CAUSE'
(Other definitions for cause that I've seen before include "Reason; belief" , "Charity - motive" , "why some people fight" , "Grounds" , "Give rise to" .)