Some medicine inside grotto nicked (5)
I believe the answer is:
tonic
'some medicine' is the definition.
(tonic is a kind of medicine)
'inside grotto nicked' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'TONIC' is hidden within 'grotto nicked'.
(Other definitions for tonic that I've seen before include "Stimulant" , "Medical restorative" , "Quinine drink; keynote" , "shot in the arm" , "Gin mixer - pick-me-up" .)