Specialised market (5)
I believe the answer is:
niche
'specialised market' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for niche that I've seen before include "small specialised market" , "Recess - alcove" , "Recess - suitable position" , "Small corner of e.g. market" , "Specialist area" .)