Work late? Male hasn't finished, certainly (4,2)
I believe the answer is:
stay up
'work late?' is the definition.
'male hasn't finished certainly' is the wordplay.
'male' becomes 'stag' (a stag is a male deer).
'hasn't finished' means to remove the last letter.
'certainly' becomes 'yup' (both can be used to answer in the affirmative).
'stag' with its last letter taken off is 'sta'.
'sta'+'yup'='STAY UP'
(Other definitions for stay up that I've seen before include "Keep late hours; avoid relegation" , "Don't go to bed - don't be relegated" , "Don't retire" , "postpone retirement" , "Decide not to retire" .)