Be quick when one is obliged to raise the net (6)
I believe the answer is:
hasten
'be quick' is the definition.
(I've seen this before)
'one is obliged to raise the net' is the wordplay.
'one is obliged' becomes 'has'.
'to raise' says the letters should be written backwards (in a down clue, letters go up).
'net' reversed gives 'ten'.
'has'+'ten'='HASTEN'
'when' acts as a link.
(Other definitions for hasten that I've seen before include "Rush" , "Speed up" , "Hurry up in old-fashioned way" , "Athens (anag.)" , "Make faster" .)