Important if knocked out of cup game (7)
I believe the answer is:
seminal
'important' is the definition.
(seminal can mean important or influential)
'if knocked out of cup game' is the wordplay.
'knocked out of' suggests deleting specific letters.
'cup game' becomes 'semi final' (match to qualify for a cup final).
'semifinal' with 'if' taken away is 'SEMINAL'.
(Other definitions for seminal that I've seen before include "Major" , "Germinal" , "Rudimentary, influential" , "Groundbreaking" , "Highly original" .)