Lock up, after a protest (6)
I believe the answer is:
assert
'protest' is the definition.
(I've seen this before)
'lock up after a' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'after' means one lot of letters go next to another.
'tress' in reverse letter order is 'ssert'.
'ssert' put after 'a' is 'ASSERT'.
(Other definitions for assert that I've seen before include "Declare, state vigorously" , "Establish control or state as true" , "Insist upon (rights etc)" , "Stares (anag)" , "Declare, claim" .)