Ostensible reason put forward to disguise the true one (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
pretext
(Other definitions for pretext that I've seen before include "Alleged reason" , "Reason put forward to conceal the true one" , "False reason, guise" , "Ostensible justification" , "Ostensible reason" .)