Stallion's back on course — whereas this one isn't (3-7)
I believe the answer is:
non-starter
'whereas this one isn't' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'stallion's back on course' is the wordplay.
'back' suggests the final letters.
'course' becomes 'starter' (course of a meal**).
The final letter of 'stallion' is 'n'.
'n'+'on'+'starter'='NON-STARTER'
(Other definitions for non-starter that I've seen before include "Someone who is never going to succeed" , "Something with no chance of success" .)