Horse carrying bishop is little one (4)
I believe the answer is:
baby
'little one' is the definition.
(thesaurus)
'horse carrying bishop' is the wordplay.
'horse' becomes 'bay' (I've seen this before**).
'carrying' indicates putting letters inside.
'bishop' becomes 'b' (abbreviation used in chess).
'bay' going around 'b' is 'BABY'.
'is' is the link.
(Other definitions for baby that I've seen before include "Newly-born child" , "Bunting" , "personal project" , "about to howl?" , "One born" .)