Has now left, but not before a bath (4)
I believe the answer is:
wash
'a bath' is the definition.
(I've seen this before)
'has now left but not before' is the wordplay.
'has now left' becomes 'was here' (was previously here but has now left).
'but not' indicates named letters should be taken away.
'before' becomes 'ere' (poetic term for 'before').
'washere' with 'ere' taken away is 'WASH'.
(Other definitions for wash that I've seen before include "Laundry" , "Rough water left by a moving boat" , "Clean; thin coat of paint" , "Stand up" , "Be credible" .)