Something eating the lettuce? Blow! (4)
I believe the answer is:
slug
I believe this clue is a double definition.
'something eating the lettuce?' is the first definition.
(slugs eat plants such as lettuce)
'blow' is the second definition.
(both can mean a hit or attack)
(Other definitions for slug that I've seen before include "Wallop" , "Some whisky perhaps" , "Bullet; swig" , "Garden pest - pellet" , "gulp!" .)