It's mightier than the sword (3)
I believe the answer is:
pen
'it's mightier than the sword' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pen that I've seen before include "'The ... is mightier than the sword'" , "Writing tool" , "International writers's organisation" , "American jug?" , "Coop" .)