Wagon string he left here for a coach (7)
I believe the answer is:
trainer
'for a coach' is the definition.
(as in a sports coach)
'wagon string he left here' is the wordplay.
I cannot really understand how this works, but
'string' could be 'train' (synonyms) and 'train' is present in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trainer that I've seen before include "Comfortable shoe" , "Soft shoe; simulator" , "Casual 11 ac" , "Sports shoe" , "Coach - one of a pair of shoes?" .)