Has pity, locking away poisoner (3)
I believe the answer is:
asp
'poisoner' is the definition.
(I've seen this before)
'has pity locking away' is the wordplay.
'locking away' indicates a hidden word.
'ASP' can be found hidden inside 'has pity'.
(Other definitions for asp that I've seen before include "A glider's what's called for" , "creeper" , "Cleopatra's supposed nemesis" , "something at the zoo" , "Snake, killed Cleopatra" .)