Look after horse's mane? (4)
I believe the answer is:
hair
'mane?' is the definition.
(thesaurus)
'look after horse's' is the wordplay.
'look' becomes 'air' (I've seen this before).
'after' means one lot of letters go next to another.
'horse' becomes 'h' ('H' can be a synonym of 'horse').
'air' after 'h' is 'HAIR'.
(Other definitions for hair that I've seen before include "Threadlike growth" , "Fur - mop" , "Filaments growing on the epidermis" , "Fibre; small extent" , "Barber's speciality; musical" .)