Knock about catapult (3)
I believe the answer is:
tap
'knock' is the definition.
(synonyms)
'about catapult' is the wordplay.
'about' says the answer is hidden in the clue.
'TAP' can be found hidden inside 'catapult'.
(Other definitions for tap that I've seen before include "Water source; take advantage of" , "British faucet" , "milk" , "Secret (phone) intercept" , "Knock gently" .)