Nasty sprain: one's swallowed painkiller (7)
I believe the answer is:
aspirin
'painkiller' is the definition.
(I know that aspirin is a type of painkiller)
'nasty sprain one's swallowed' is the wordplay.
'nasty' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'swallowed' is an insertion indicator.
'sprain' is an anagram of 'aspirn'.
'aspirn' going around 'i' is 'ASPIRIN'.
(Other definitions for aspirin that I've seen before include "Pain-relieving drug made in Paris" , "Sip rain (anag.)" , "Pain-relief drug" , "number" , "Anodyne" .)