Tie up about fifty unable to see (5)
I believe the answer is:
blind
'unable to see' is the definition.
(I've seen this before)
'tie up about fifty' is the wordplay.
'tie up' becomes 'bind' (synonyms).
'about' means one lot of letters goes inside another.
'fifty' becomes 'L' (Roman numeral).
'bind' placed around 'l' is 'BLIND'.
(Other definitions for blind that I've seen before include "Visually challenged" , "In the dark" , "Unobserving" , "Sightless" , "deception" .)