Oppose wild jumping Jack abandons (6)
I believe the answer is:
impugn
'oppose' is the definition.
(thesaurus)
'wild jumping jack abandons' is the wordplay.
'wild' indicates an anagram.
'jack' becomes 'J' (playing card abbreviation).
'abandons' suggests deleting specific letters.
'jumping' with 'j' removed is 'umping'.
'umping' anagrammed gives 'IMPUGN'.
(Other definitions for impugn that I've seen before include "Attack as false, challenge" , "Dispute the truth of" , "Call into question; criticise" , "Challenge (e.g. honesty)" , "Dispute honesty of (someone's motive)" .)