Eleven on the shelf? (5)
I believe the answer is:
aside
'eleven on the shelf?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'eleven on the shelf?' is the wordplay.
'eleven' becomes 'side' (a football side).
'on' says to put letters next to each other.
'the shelf?' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'side' after 'a' is 'ASIDE'.
Can you help me to learn more?
(Other definitions for aside that I've seen before include "In reserve; actor's remark to audience" , "(Pushed) out of the way" , "Quiet words spoken" , "secretly" , "Separately" .)