Errol, disheartened, quits side (6)
I believe the answer is:
'side' is the definition.
(I've seen this before)
'errol disheartened quits' is the wordplay.
'disheartened' suggests removing the centre.
'quits' becomes 'even' (as in 'call it quits').
'errol' with its middle taken out is 'el'.
(Other definitions for eleven that I've seen before include "Time for the mid-morning tea break?", "Players", "Not first team (6,6) - See 24", "Football team; number", "Time for morning coffee".)