A light blow may be apt (3)
I believe the answer is:
tap
'a light blow' is the definition.
(I've seen this in another clue)
'may be apt' is the wordplay.
'may be' indicates anagramming the letters.
'apt' is an anagram of 'TAP'.
(Other definitions for tap that I've seen before include "Knock gently" , "Light touch (hot or cold?)" , "In America, it's a spigot" , "It's faucet in the US" , "Light blow; draw sap from (tree)" .)