Vigorously contest red card, but acknowledge one's error (5,9)
I believe the answer is:
stand corrected
'acknowledge one's error' is the definition.
('I stand corrected' means 'I acknowledge my error')
'vigorously contest red card' is the wordplay.
'vigorously' indicates an anagram (the letters move vigorously around).
'contest'+'red'+'card'='contestredcard'
'contestredcard' with letters rearranged gives 'STAND CORRECTED'.
'but' is the link.
(Other definitions for stand corrected that I've seen before include "unable to sit after six of the best?" , "Admit error" , "Admit mistake" .)