Discover wobble in car seat (9)
I believe the answer is:
ascertain
'discover' is the definition.
(I know this)
'wobble in car seat' is the wordplay.
'wobble' indicates an anagram (letters wobble around).
'in'+'car'+'seat'='incarseat'
'incarseat' with letters rearranged gives 'ASCERTAIN'.
(Other definitions for ascertain that I've seen before include "Prove" , "Get to know" , "Find out - make sure" , "Become aware" , "Establish (the facts)" .)