Maintain a lock must be sent back (6)
I believe the answer is:
assert
'maintain' is the definition.
('assert' can be a synonym of 'maintain')
'a lock must be sent back' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'must be sent back' says the letters should be written backwards.
'tress' backwards is 'ssert'.
'a'+'ssert'='ASSERT'
(Other definitions for assert that I've seen before include "Insist upon (rights etc)" , "Declare or affirm" , "Confidently say" , "Maintain - allege" , "State firmly, aver" .)