If sent off, he finishes up early, perhaps (6)
I believe the answer is:
player
'if sent off he finishes' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'up early perhaps' is the wordplay.
'up' becomes 'p' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'perhaps' indicates an anagram.
'early' anagrammed gives 'layer'.
'p'+'layer'='PLAYER'
Can you help me to learn more?
(Other definitions for player that I've seen before include "South African golfer, b. 1935" , "Person who takes part in a game" , "Important participant" , "Active sportsperson" , "Professional musician - team sportsperson" .)