Youngster father held back, seizing catapult? On the contrary (7)
I believe the answer is:
sapling
'youngster' is the definition.
(thesaurus)
'catapult? on the contrary' is the wordplay.
'catapult?' becomes 'sling' ('sling' can be a synonym of 'catapult').
'on' is an insertion indicator (as in clothing 'on' a person).
'the contrary' becomes 'ap' (I can't justify this - if you can you should believe this answer much more).
'sling' enclosing 'ap' is 'SAPLING'.
'father held back seizing' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sapling that I've seen before include "Little tree" , "Poetic youth" , "Baby tree" , "Young slender tree" , "small bit of wood" .)