Valuable box certain to be found among the crates at sea (8,5)
I believe the answer is:
treasure chest
'valuable box' is the definition.
'treasure chest' can be an answer for 'box' (treasure chest is a kind of box). I am not sure about the 'valuable' bit.
'certain to be found among the crates at sea' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to be found among' means one lot of letters goes inside another.
'at sea' indicates an anagram.
'the'+'crates'='thecrates'
'thecrates' with letters rearranged gives 'treachest'.
'sure' placed within 'treachest' is 'TREASURE-CHEST'.
Can you help me to learn more?
(Other definitions for treasure chest that I've seen before include "Box of wealth" , "Box for valuables" , "Box holding items of great value" , "Store of valuable items" , "Typical pirate hoard" .)