Lead in flue depressed sweep (4)
I believe the answer is:
flow
'sweep' is the definition.
(thesaurus)
'lead in flue depressed' is the wordplay.
'lead in' suggests taking the first letters.
'depressed' becomes 'low' (similar in meaning).
The initial letter of 'flue' is 'f'.
'f'+'low'='FLOW'
(Other definitions for flow that I've seen before include "Run or move like a liquid" , "Steady continuous stream" , "Well forth" , "Wolf (anag.)" , "Scapa - , Orkney anchorage" .)