Claim a lock is upside down (6)
I believe the answer is:
assert
'claim' is the definition.
(thesaurus)
'a lock is upside down' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'is upside down' says the letters should be written backwards.
'tress' in reverse letter order is 'ssert'.
'a'+'ssert'='ASSERT'
(Other definitions for assert that I've seen before include "Declare, profees" , "State firmly and with conviction" , "State confidently" , "Declare, profess" , "Stares (anag)" .)