Giant brown boxes appeal (5)
I believe the answer is:
titan
'giant' is the definition.
(synonyms)
'brown boxes appeal' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'appeal' becomes 'it'.
'tan' enclosing 'it' is 'TITAN'.
(Other definitions for titan that I've seen before include "primordial god" , "Giant of Greek mythology" , "Goddess" , "Rhea, perhaps" , "Colossus" .)