Shoe for a coach (7)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'shoe' is the first definition.
(I've seen this before)
'a coach' is the second definition.
'for' is the link.
(Other definitions for trainer that I've seen before include "Running shoe", "He assists athlete to become fit", "Coach - one of a pair of shoes?", "Shoe; instructor", "Sports shoe", "Casual shoe", "The odd thing an athlete may leave behind", "Person in charge of stable", "Instructor; shoe" and "Machine that simulates the conditions of flying a plane".)