Plenty of hard biscuit aboard ship! (6)
I believe the answer is:
stacks
'plenty' is the definition.
(thesaurus)
'hard biscuit aboard ship' is the wordplay.
'hard biscuit' becomes 'tack' (I can't explain this - if you can you should give a lot more credence to this answer).
'aboard' is an insertion indicator.
'ship' becomes 'ss'.
'tack' placed inside 'ss' is 'STACKS'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for stacks that I've seen before include "A lot (of hay?)" , "Large tall chimneys" , "Orderly heaps as of hay" , "Orderly piles or heaps" , "Ricks" .)