Something on mast gets snake back in work (7)
I believe the answer is:
topsail
'something on mast' is the definition.
I can't tell whether this defines the answer.
'snake back in work' is the wordplay.
'snake back' becomes 'psa' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'work' becomes 'toil' (toil is a kind of work).
'psa' placed inside 'toil' is 'TOPSAIL'.
'gets' acts as a link.
Can you help me to learn more?
(Other definitions for topsail that I've seen before include "Highly-placed canvas" , "It's set high up on a sailing ship" , "Upper piece of ship's canvas" , "Part of a ship's rigging" , "Uppermost attachment of a vessel's rigging" .)