Boil over with umpire finally at game point (5)
I believe the answer is:
erupt
'boil over' is the definition.
(I've seen this in another clue)
'umpire finally at game point' is the wordplay.
'finally' suggests the final letters.
'at' says to put letters next to each other.
'game' becomes 'RU' (rugby union).
'point' becomes 'pt' (abbreviation for point).
The final letter of 'umpire' is 'e'.
'e'+'ru'+'pt'='ERUPT'
'with' is the link.
(Other definitions for erupt that I've seen before include "Break out suddenly and violently" , "be active" , "Discharge" , "Suddenly appear" , "Break out dramatically" .)