Stuff from pharmacy sterilised liquid-filled cavity (4)
I believe the answer is:
cyst
'liquid-filled cavity' is the definition.
(type of cavity in or on the body)
'stuff from pharmacy sterilised' is the wordplay.
'stuff' indicates a hidden word (I've seen 'stuffing' mean this).
'CYST' is hidden in the letters of 'from pharmacy sterilised'.
(Other definitions for cyst that I've seen before include "Blister" , "Bladder" , "Benign growth" , "Small sac in body (that may form abnormal lump)" , "Abnormal body cavity" .)