In favour of leave but do without (5)
I believe the answer is:
forgo
'do without' is the definition.
(I've seen this in another clue)
'in favour of leave' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'leave' becomes 'go' (to go is to depart or leave).
'for'+'go'='FORGO'
'but' acts as a link.
(Other definitions for forgo that I've seen before include "Sacrifice" , "Waive" , "pass up" , "Relinquish, give up" , "Go without (something)" .)