Trigger one to come in and countersign it eventually (6)
I believe the answer is:
ignite
'trigger' is the definition.
'one to come in and countersign it eventually' is the wordplay.
'one to come in' indicates the answer is hidden within the clue (I've seen 'coming in' mean this).
'IGNITE' is hidden in the letters of 'and countersign it eventually'.
(Other definitions for ignite that I've seen before include "Burn" , "'Set alight, fire up (6)'" , "Torch" , "Set fire to, light up" , "Catch fire; provoke" .)