Resin in his apartment (3)
I believe the answer is:
sap
'resin' is the definition.
(I know that sap is a type of plant resin)
'in his apartment' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SAP' can be found hidden inside 'his apartment'.
(Other definitions for sap that I've seen before include "Juice; energy" , "Plant liquid or general vitality" , "Undermine; fool" , "Charlie" , "Drain - vital fluid" .)