Horse in agreeable position (5)
I believe the answer is:
niche
'position' is the definition.
(niche is a kind of position)
'horse in agreeable' is the wordplay.
'horse' becomes 'h' (synonyms).
'in' indicates putting letters inside.
'agreeable' becomes 'nice' (I've seen this before).
'h' placed within 'nice' is 'NICHE'.
(Other definitions for niche that I've seen before include "Select place in market" , "small specialised market" , "Chine (anag) - suitable place in life" , "good spot!" , "Recess - suitable position" .)