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 "Football side" , "Lowest two-digit prime" , "Age to start secondary education" , "Smallest positive three-syllable integer" , "sports team?" .)