Advance money to sailor, say, for sandwich (9)
I believe the answer is:
submarine
'for sandwich' is the definition.
(submarine sandwich is a kind of sandwich)
'advance money' is the wordplay.
'advance' becomes 'sub' (I've seen this in other clues).
'money' becomes 'marine' (I can't explain this - if you can you should believe this answer much more).
'sub'+'marine'='SUBMARINE'
'to sailor say' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for submarine that I've seen before include "... underwater craft" , "Under the surface" , "Under the sea" , "Submersible warship" , "snack" .)