Rejection of umpire by American league (7)
I believe the answer is:
refusal
'rejection' is the definition.
(I've seen this in another clue)
'umpire by american league' is the wordplay.
'umpire' becomes 'ref' (short for referee).
'by' says to put letters next to each other.
'american' becomes 'usa'.
'league' becomes 'l' (abbreviation).
'ref'+'usa'+'l'='REFUSAL'
'of' is the link.
(Other definitions for refusal that I've seen before include "Negative response" , "First this is first choice before others" , "Non-acceptance" , "regrets?" , "Horse's action in declining to jump a fence" .)