I start to grimace a lot — a nasty sort of pain (7)
I believe the answer is:
otalgia
'sort of pain' is the definition.
'i start to grimace a lot a nasty' is the wordplay.
'start to' suggests taking the first letters.
'nasty' indicates anagramming the letters.
The initial letter of 'grimace' is 'g'.
'i'+'g'+'a'+'lot'+'a'='igalota'
'igalota' anagrammed gives 'OTALGIA'.
(Other definitions for otalgia that I've seen before include "pain!" , "Earache" , "painful condition" , "Problem with ears" , "Ear ache" .)