Willing to participate, gave out in a ring after heavyweight’s entrance (4-1-2)

'willing to participate' is the definition.
(have-a-go can mean willing to try something)

'gave out in a ring after heavyweight's entrance' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'in' is an insertion indicator.
'ring' becomes 'o' ().
'after' is a charade indicator (letters next to each other).
'entrance' indicates taking the first letters.
The initial letter of 'heavyweights' is 'h'.
'gave' is an anagram of 'veag'.
'a'+'o'='ao'
'veag' inserted inside 'ao' is 'aveago'.
'aveago' after 'h' is 'HAVE-A-GO'.