Interval in cup match enthralling millions (4)
I believe the answer is:
time
'interval' is the definition.
(synonyms)
'cup match enthralling millions' is the wordplay.
'cup match' becomes 'tie'.
'enthralling' is an insertion indicator.
'millions' becomes 'm' (abbreviation).
'tie' enclosing 'm' is 'TIME'.
'in' acts as a link.
(Other definitions for time that I've seen before include "Hours clocked up?" , "Finite duration" , "Prison sentence; occasion" , "Age" , ""The enemy"" .)