Throw up on parents' back seat (4)
I believe the answer is:
spew
'throw up' is the definition.
(I know that throw up can be written as spew)
'on parents' back seat' is the wordplay.
'on' means one lot of letters go next to another.
'back' says to take the final letters (the letter at the back).
'seat' becomes 'pew' (pew is a kind of seat).
The last letter of 'parents' is 's'.
's'+'pew' is 'SPEW'.
(Other definitions for spew that I've seen before include "Casst out in a stream" , "Spit or stream out" , "bring it up" , "Erupt, eject" , "Gush forth, vomit" .)