Wise having drain that is not emptied? (7)
I believe the answer is:
sapient
'wise' is the definition.
('sapient' can be similar in meaning to 'wise')
'drain that is not emptied?' is the wordplay.
'drain' becomes 'sap' (sapping is a kind of draining).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'emptied?' suggests removing the centre.
'not' with its middle taken out is 'nt'.
'sap'+'ie'+'nt'='SAPIENT'
'having' is the link.
(Other definitions for sapient that I've seen before include "Having intelligence" , "Literary word for wise" , "very keen" , "Acutely insightful and wise" , "Snipe at, being wise" .)