Giant disguised in tat (5)
I believe the answer is:
titan
'giant' is the definition.
(titans are giant mythological creatures)
'disguised in tat' is the wordplay.
'disguised' indicates an anagram.
'in'+'tat'='intat'
'intat' with letters rearranged gives 'TITAN'.
(Other definitions for titan that I've seen before include "Atlas, possibly" , "Colossus" , "Taint in a great man" , "One of the first pantheon of Greek deities" , "A son of Uranus and Gaea" .)