Insist upon a lock being put back (6)
I believe the answer is:
assert
'insist' is the definition.
(asserting is a kind of insisting)
'upon a lock being put back' is the wordplay.
'upon' says to put letters next to each other.
'lock' becomes 'tress' (both can mean a piece of hair).
'being put back' is a reversal indicator.
'a' after 'tress' is 'tressa'.
'tressa' written backwards gives 'ASSERT'.
(Other definitions for assert that I've seen before include "Establish control or state as true" , "Declare, claim" , "Maintain formally" , "protest" , "Contend, maintain" .)