Model returning to collect bronze colossi (6)
I believe the answer is:
titans
'colossi' is the definition.
(I know this)
'model returning to collect bronze' is the wordplay.
'model' becomes 'sit' (e.g. for a painting).
'returning' says the letters should be written backwards.
'to collect' means one lot of letters goes inside another (inserted letters are collected by others).
'bronze' becomes 'tan' (both can mean to get a sun tan).
'sit' reversed gives 'tis'.
'tis' going around 'tan' is 'TITANS'.
(Other definitions for titans that I've seen before include "Taints in giant men" , "Elder deities of Greek myth" , "Mythological beings of great size and strength" , "Giant gods of Greek mythology" , "Taints in mythical giants" .)