Hide large treasure boxes (4)
I believe the answer is:
pelt
'hide' is the definition.
(synonyms)
'large treasure boxes' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'treasure' becomes 'pet' (both are terms of endearment).
'boxes' is an insertion indicator (the outside letters must 'box' inside letters in).
'l' put into 'pet' is 'PELT'.
(Other definitions for pelt that I've seen before include "Skin - bombard - hurry!" , "Skin; rain hard" , "Run fast - rain hard" , "Hairy coat of a mammal" , "Skin, hide" .)