Ship rocks on wave (10)
I believe the answer is:
icebreaker
'ship' is the definition.
(icebreaker is a kind of ship)
'rocks on wave' is the wordplay.
'rocks' becomes 'ice' (both can informally mean diamonds).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'wave' becomes 'breaker' (breaker is a kind of wave).
'ice'+'breaker'='ICEBREAKER'
(Other definitions for icebreaker that I've seen before include "which opens dialogue?" , "Reinforced vessel" , "useful at parties" , "Something relieving initial tension" , "Chatty partygoer without reserve" .)