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 anagramming the letters.
'jack' becomes 'J' (playing card abbreviation).
'abandons' indicates named letters should be taken away.
'jumping' with 'j' taken away is 'umping'.
'umping' is an anagram of '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)" .)