Ostensible reason (7)
I believe the answer is:
pretext
'ostensible reason' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pretext that I've seen before include "False reason as justification for an action" , "Reason put forward to conceal the true one" , "False reason, guise" , "Justification, excuse" , "Ostensible reason" .)