Test the writer’s reason for action (6)
I believe the answer is:
motive
'reason for action' is the definition.
(what motivates something)
'test the writer's' is the wordplay.
'test' becomes 'mot' (as in an MOT test for a vehicle).
'the writer's' becomes 'ive' ('I have' could mean 'the writer has').
'mot'+'ive'='MOTIVE'
(Other definitions for motive that I've seen before include "Underlying intention" , "Purpose in doing something" , "Reason for doing something, perhaps a crime" , "Purpose - reason" , "Ground" .)