You may find them by the sea and on board ship (5)
I believe the answer is:
sands
'you may find them by the sea' is the definition.
I can't tell whether this definition defines the answer.
'and on board ship' is the wordplay.
'on board' indicates putting letters inside (inserted letters are taken on board).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'and' going inside 'ss' is 'SANDS'.
Can you help me to learn more?
(Other definitions for sands that I've seen before include "Smooths" , "They run out (of time)" , "Rubs smooth" , "Beaches" , "The Riddle of the . . . . . (novel by Erskine Childers)" .)