One binder in a row? (4)
I believe the answer is:
tier
'a row?' is the definition.
(a tier is a row of things)
'one binder in' is the wordplay.
'one' becomes 'i' (Roman numeral).
'binder' becomes 'ter' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'i' inserted inside 'ter' is 'TIER'.
Can you help me to learn more?
(Other definitions for tier that I've seen before include "band" , "this person secures" , "Hierarchical level" , "Row - one forming a knot" , "One who secures" .)