It turned brown for a giant (5)
I believe the answer is:
titan
'for a giant' is the definition.
('titan' can be a synonym of 'giant')
'it turned brown' is the wordplay.
'turned' says the letters should be written backwards.
'brown' becomes 'tan' (tan is a kind of brown).
'it' back-to-front is 'ti'.
'ti'+'tan'='TITAN'
(Other definitions for titan that I've seen before include "An old Greek god - Atlas was one" , "A giant; Saturn moon" , "colossal one" , "Saturn's moon" , "Taint in old Greek god" .)