Maintain a lock-up (6)
I believe the answer is:
assert
'maintain' is the definition.
(I know that maintain can be written as assert)
'a lock-up' is the wordplay.
'lock' becomes 'tress' (both can mean a piece of hair).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'tress' written backwards gives 'ssert'.
'a'+'ssert'='ASSERT'
(Other definitions for assert that I've seen before include "State firmly or vigorously" , "Declare, profess" , "Contend, maintain" , "Confidently say" , "protest" .)