It's clear a chemist's controlling pain (7)
I believe the answer is:
earache
'pain' is the definition.
(earache is a kind of pain)
'it's clear a chemist's controlling' is the wordplay.
'controlling' indicates a hidden word (the solution is controlled or contained).
'EARACHE' is hidden in the letters of 'ts clear a chemists'.
(Other definitions for earache that I've seen before include "listener's discomfort" , "Ace hare (anag.)" , "result of nagging?" , "Lughole pain" , "Incessant complaining" .)