Nursery horse kept in different line to nag (6)
I believe the answer is:
niggle
'to nag' is the definition.
(thesaurus)
'nursery horse kept in different line' is the wordplay.
I cannot really see how this works, but
'horse' could be 'gg' (geegee or GG is a childish term for a horse) and 'gg' is present in the answer.
an anagram of 'line' is 'nile' which is located in the left over letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for niggle that I've seen before include "Petty annoyance" , "Cause slight annoyance" , "Persistently irritate" , "Carp" , "Minor criticism - slight worry" .)