One who covers costs when player's left out (5)
I believe the answer is:
payer
'one who covers costs' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'player's left out' is the wordplay.
'left' becomes 'l' (common abbreviation).
'out' is a deletion indicator.
'player' with 'l' taken out is 'PAYER'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for payer that I've seen before include "Remitter" , "One foots the bill" , "One handing over money" , "One footing the bill" , "Someone to fund" .)