Another player, a robotic sort, not extremely cheery (4)
I believe the answer is:
borg
'another player' is the definition.
'Borg' can be an answer for 'player' (tennis player Bjorn Borg). I am not sure about the 'another' bit.
'a robotic sort not extremely cheery' is the wordplay.
'a robotic sort' becomes 'cyborg' (person with machine body parts).
'not extremely' means to look at the middle letters (remove the letters on the extremes of the word).
'cheery' becomes 'rg' (I can't justify this - if you can you should give a lot more credence to this answer).
The central letters of 'cyborg' are 'bo'.
'bo'+'rg'='BORG'
(Another definition for borg that I've seen is " Bjorn ...., once a great tennis player".)