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' means one lot of letters goes inside another (the outside letters must 'box' inside letters in).
'l' going within '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" .)