Test I have for a reason (6)
I believe the answer is:
motive
'a reason' is the definition.
(thesaurus)
'test i have' is the wordplay.
'test' becomes 'mot'.
'i have' becomes 'ive' (**).
'mot'+'ive'='MOTIVE'
'for' acts as a link.
(Other definitions for motive that I've seen before include "Rationale, purpose" , "Reason for doing something, say murder" , "What induces one to take a certain action" , "Reason for committing a crime" , "Underlying purpose" .)