Liquid discharged in switch-over period (9)
I believe the answer is:
watershed
'switch-over period' is the definition.
(as in the TV watershed)
'liquid discharged' is the wordplay.
'liquid' becomes 'water' (water is a kind of liquid).
'discharged' becomes 'shed' (shed can mean to discharge or remove).
'water'+'shed'='WATERSHED'
'in' acts as a link.
(Other definitions for watershed that I've seen before include "dividing line" , "Event marking a turning point" , "Line of high land from which streams flow in opposite directions" , "It's OK to swear after this" , "Time after which adult programmes may be shown" .)