A cat is off course (6)
I believe the answer is:
astray
'off course' is the definition.
(to be led astray is to be led off course)
'a cat' is the wordplay.
'cat' becomes 'stray' (stray is a kind of cat).
'a'+'stray'='ASTRAY'
'is' is the link.
(Other definitions for astray that I've seen before include "Off target" , "(Gone) in wrong direction" , "Off one's route or lost" , "Philip -, English twentieth century poet" , "Missing, lost" .)