Maintain there's a lock on the way up (6)
I believe the answer is:
assert
'maintain' is the definition.
('assert' can be a synonym of 'maintain')
'there's a lock on the way up' is the wordplay.
'there's' says to put letters next to each other.
'lock' becomes 'tress' (I've seen this before).
'on the way up' shows that the letters should be reversed in order.
'tress' written backwards gives 'ssert'.
'a' put next to 'ssert' is 'ASSERT'.
(Other definitions for assert that I've seen before include "Declare, profees" , "Contend, maintain" , "Swear" , "protest" , "Establish control or state as true" .)