Tin item on chest supported by ship (5)
I believe the answer is:
brass
'tin' is the definition.
(thesaurus)
'item on chest supported by ship' is the wordplay.
'item on chest' becomes 'bra' (I can't explain this - if you can you should give a lot more credence to this answer).
'supported by' is a charade indicator (letters next to each other) (in a down clue, some letters hold up others).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'bra'+'ss'='BRASS'
Can you help me to learn more?
(Other definitions for brass that I've seen before include "Nerve (but not of steel!)" , "Metal - cheek" , "Yellowish alloy" , "Alloy of Cu and Zn" , "some musicians!" .)