Favour one eleven, perhaps, to bag boundaries (4,5)
I believe the answer is:
take sides
'favour one eleven perhaps' is the definition.
I can't tell whether this defines the answer.
'bag boundaries' is the wordplay.
'bag' becomes 'take' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'boundaries' becomes 'sides' (side is a kind of boundary).
'take'+'sides'='TAKE SIDES'
'to' is the link.
Can you help me to learn more?
(Other definitions for take sides that I've seen before include "Support one faction" , "Be partisan, support one party" , "Join the struggle" , "Support one of parties in dispute" , "Prefer a party" .)