Trim a lawn (3)
I believe the answer is:
mow
'trim a lawn' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mow that I've seen before include "Cut grass or grain" , "Cut (e.g. grass)" , "'Cut down, hay, say (3)'" , "Cut down, usually grass" , "Cut grass on lawn" .)