Advance crew member for sandwich (9)
I believe the answer is:
submarine
'for sandwich' is the definition.
(submarine is a kind of sandwich)
'advance crew member' is the wordplay.
'advance' becomes 'sub' (both can mean a loan).
'crew member' becomes 'marine' (I can't justify this - if you can you should believe this answer much more).
'sub'+'marine'='SUBMARINE'
Can you help me to learn more?
(Other definitions for submarine that I've seen before include "Under the sea" , "Captain Nemo's Nautilus" , "... underwater craft" , "snack" , "U-boat" .)