Quickly look over small defender in reserves (5,4)

'reserves' is the definition.
The answer and definition can be both to do with ownership as well as being verbs in their -s form.
Maybe there's an association between them I don't understand?

'quickly look over small defender' is the wordplay.
'quickly look over' becomes 'keep' (I am not sure about this - if you are sure you should believe this answer much more).
'small' becomes 's' (abbreviation - e.g. clothes size).
'defender' becomes 'back' (left-backs and right-backs are football defenders).
'keep'+'s'+'back'='KEEPS BACK'

'in' is the link.

Can you help me to learn more?