Exceptional whistle-blower is great on turf (10)
I believe the answer is:
supergrass
'exceptional whistle-blower' is the definition.
(prolific police informer)
'great on turf' is the wordplay.
'great' becomes 'super' (I've seen this before).
'on' says to put letters next to each other.
'turf' becomes 'grass' (I've seen this before).
'super'+'grass'='SUPERGRASS'
'is' acts as a link.
(Other definitions for supergrass that I've seen before include "Repentant criminal perhaps" , "Large-scale informer" , "He does a lot of shopping" , "Very important police informer" .)