Fourth and fifth in racecourse moving to the front — one's a colt (7)
I believe the answer is:
trainee
'colt' is the definition.
(colt can informally mean a new recruit)
'fourth and fifth in racecourse moving to the front one's a' is the wordplay.
I cannot really understand how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'a' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
The remaining letters 'tree' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for trainee that I've seen before include "One being instructed" , "Person being taught" , "one needs to practice" , "A beginner in Aintree" , "One taking instruction" .)