Necessary restraint shown in whack on bum (4,4)
I believe the answer is:
seat belt
'necessary restraint' is the definition.
'seat-belt' can be an answer for 'restraint' (seat-belt is a kind of restraint). I am not certain of the 'necessary' bit.
'whack on bum' is the wordplay.
'whack' becomes 'belt' (I've seen this in another clue).
'on' says to put letters next to each other.
'bum' becomes 'seat' (synonyms).
'belt' after 'seat' is 'SEAT-BELT'.
'shown in' is the link.
Can you help me to learn more?
(Other definitions for seat belt that I've seen before include "Car safety feature" , "Safety strap worn in car" , "Safety device in car - wearing mandatory" , "Protective strap in car" , "Car safety strap (4.4)" .)