...required a lens fixed with ties (9)
I believe the answer is:
essential
'required' is the definition.
(I've seen this in another clue)
'a lens fixed with ties' is the wordplay.
'fixed with' indicates anagramming the letters.
'a'+'lens'='alens'
'alens' anagrammed with 'ties' makes 'ESSENTIAL'.
(Other definitions for essential that I've seen before include "as oil might be" , "something necessary" , "Indispensible" , "'Absolutely necessary, required (9)'" , "absolutely neccessary?" .)