Spill a secret (4)
I believe the answer is:
blab
'spill a secret' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for blab that I've seen before include "Talk indiscreetly" , "Let slip" , "Tell tales" , "Shoot one's mouth off" , "Let the cat out of the bag" .)