Falls about: sailor to do something (8)
I believe the answer is:
cataract
'falls' is the definition.
('cataract' can be a synonym of 'falls')
'about sailor to do something' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'sailor' becomes 'tar' (tar is a kind of sailor).
'to' means one lot of letters go next to another.
'do something' becomes 'act' (I've seen this before).
'ca'+'tar'+'act'='CATARACT'
(Other definitions for cataract that I've seen before include "what may have a bad effect on the viewer" , "Cascade; eye defect" , "It may prevent someone seeing" , "(Nile) waterfall" , "Lens defect" .)