Tim has one left, back at border (5)
I believe the answer is:
limit
'border' is the definition.
(I've seen this in another clue)
'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' shows that the letters should be reversed in order.
'tim'+'i'+'l'='timil'
'timil' reversed gives 'LIMIT'.
'at' acts as a link.
(Other definitions for limit that I've seen before include "Bound, restriction" , "Point beyond which one should not go" , "control" , "Breaking point" , "Frontier" .)