Say too much (4)
I believe the answer is:
blab
'say too much' 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 "Give the game away" , "Shoot one's mouth off" , "Spill the beans" , "Let the cat out of the bag" , "Talk indiscreetly" .)