A lock-up, say (6)
I believe the answer is:
assert
'say' is the definition.
(thesaurus)
'a lock-up' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'tress' backwards is 'ssert'.
'a'+'ssert'='ASSERT'
(Other definitions for assert that I've seen before include "Aver, declare" , "Insist upon (rights etc)" , "Declare, profees" , "State firmly and positively" , "State clearly" .)