Sure to be tied up (5)
I believe the answer is:
bound
This clue is a double definition.
'sure' is the first definition.
(synonyms)
'be tied up' is the second definition.
(I've seen this in another clue)
'to' acts as a link.
(Other definitions for bound that I've seen before include "Destined" , "Bob < sure < to be heading for" , "Jump - certain - limit" , "Place restrictions" , "Confined" .)