'Stern Bill' -- the bloke's a real pain (8)
I believe the answer is:
backache
'a real pain' is the definition.
'stern bill the bloke's' is the wordplay.
'stern' becomes 'back' (I've seen this before).
'bill' becomes 'ac'.
'the bloke' becomes 'he'.
'back'+'ac'+'he'='BACKACHE'
(Other definitions for backache that I've seen before include "Eg, lumbar pain" , "Eg, lumbago" , "Pain in rear of body" , "Spinal pain" , "common complaint" .)