Organisation providing a particular service (6)
I believe the answer is:
agency
'organisation providing a particular service' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for agency that I've seen before include "Office or business of one who does business for others" , "Government office" , "Travel ..., advertising..., employment" , "Business providing specific service" , "Government unit - business providing services" .)