Bad sprain? Take one tablet (7)
I believe the answer is:
'tablet' is the definition.
(tablet of aspirin medication)
'bad sprain? take one' is the wordplay.
'bad' indicates an anagram.
'take' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'sprain' anagrammed gives 'aspirn'.
'aspirn' going around 'i' is 'ASPIRIN'.
(Other definitions for aspirin that I've seen before include "Headache pill" , "Anodyne" , "Anti-inflammatory drug" , "Headache killer?" , "Anti-inflammatory tablet" .)