It’s safe to tie up (6)
I believe the answer is:
secure
I believe this clue is a double definition.
'it's safe' is the first definition.
(similar in meaning)
'tie up' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for secure that I've seen before include "Safe, free from danger" , "Safe against attack or intrusion" , "Firmly fixed or free from uncertainty" , "Peg" , "Free from danger and financially sound" .)