About to employ reason (5)
I believe the answer is:
cause
'reason' is the definition.
(I've seen this in another clue)
'about to employ' is the wordplay.
'about' becomes 'ca' (circa).
'to 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" .)