Tim has one left back at boundary (5)
I believe the answer is:
limit
'boundary' is the definition.
(I know that boundary can be written as limit)
'tim has one left back' is the wordplay.
'has' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'left' becomes 'L' (common abbreviation).
'back' says the letters should be written backwards.
'tim'+'i'+'l'='timil'
'timil' backwards is 'LIMIT'.
'at' is the link.
(Other definitions for limit that I've seen before include "Confine within bounds" , "Constrain" , "Frontier" , "Constraint" , "verge" .)