Coppers partially open cell (5)
I believe the answer is:
pence
'coppers' is the definition.
(I've seen this before)
'partially open cell' is the wordplay.
'partially' indicates the answer is hidden within the clue.
'PENCE' can be found hidden inside 'open cell'.
(Other definitions for pence that I've seen before include "Copper coins" , "Little coins" , "President's vice" , "offering to St Peter" , "Small-value coins" .)