Wild horse has to, followed by trained nag (7)
I believe the answer is:
mustang
'wild horse' is the definition.
(I've seen this before)
'has to followed by trained nag' is the wordplay.
'has to' becomes 'must' (I've seen this before).
'followed by' means one lot of letters go next to another.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'nag' anagrammed gives 'ang'.
'must'+'ang'='MUSTANG'
(Other definitions for mustang that I've seen before include "Wild or half-wild horse" , "Small feral horse of the US" , "Wild horse of the West" , "creature" , "Small, wild US horse" .)