Cup tie, if lost, will be momentous (7)
I believe the answer is:
seminal
'momentous' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
'cup tie if lost' is the wordplay.
'cup tie' becomes 'semi' (semi-final is a type of cup tie).
'if' becomes 'na' (I can't justify this - if you can you should give a lot more credence to this answer).
'lost' becomes 'l' (abbreviation in sporting league tables).
'semi'+'na'+'l'='SEMINAL'
'will be' is the link.
Can you help me to learn more?
(Other definitions for seminal that I've seen before include "Strongly influencing future developments" , "Of seed, or giving rise to new developments" , "Very important, original" , "Strongly influential" , "Highly original" .)