Mate's standard lease is up (7)
I believe the answer is:
partner
'mate's' is the definition.
('partner' can be a synonym of 'mate')
'standard lease is up' is the wordplay.
'standard' becomes 'par' (I've seen this before).
'lease' becomes 'rent' ('rent' can be a synonym of 'lease').
'is up' says the letters should be written in reverse (in down clue: letters go upwards).
'rent' in reverse letter order is 'tner'.
'par'+'tner'='PARTNER'
(Other definitions for partner that I've seen before include "Either member of a couple" , "Sidekick" , "Professional colleague" , "Dancing companion" , "south to north, perhaps" .)