Acute pain (5)
I believe the answer is:
smart
I believe this clue is a double definition.
'acute' is the first definition.
(thesaurus)
'pain' is the second definition.
(smart is a kind of pain)
(Other definitions for smart that I've seen before include "Sting - witty" , "Quick; tidy; sting" , "Brainy" , "Neat - clever" , "Clever or stylish" .)