Substitutes on last of cup ties (8)
I believe the answer is:
replaces
'substitutes' is the definition.
('replace' can be a synonym of 'substitute')
'on last of cup ties' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to'**).
'last of' suggests the final letters.
'ties' becomes 'laces' (lacing is a kind of tying).
The last letter of 'cup' is 'p'.
're'+'p'+'laces'='REPLACES'
(Other definitions for replaces that I've seen before include "Substtitutes, supplants" , "Substitutes, supplants" , "Puts back" , "Substitutes one for another" .)