Umpire, 50, going round America getting knock-back (7)
I believe the answer is:
refusal
'knock-back' is the definition.
I can't judge whether this defines the answer.
'umpire 50 going round america' is the wordplay.
'umpire' becomes 'ref' (short for referee).
'50' becomes 'l' (Roman numeral).
'going round' indicates putting letters inside (some letters go around others).
'america' becomes 'usa' (United States of America).
'ref'+'l'='refl'
'refl' going around 'usa' is 'REFUSAL'.
'getting' acts as a link.
Can you help me to learn more?
(Other definitions for refusal that I've seen before include "Non-acceptance" , "opportunity to accept or decline" , "Dissent" , "Horse's action in declining to jump a fence" , "The act of saying no" .)