For a great man, it's wrong to upset Nat (5)
I believe the answer is:
titan
'for a great man' is the definition.
(I've seen this before)
'it's wrong to upset nat' is the wordplay.
'wrong' indicates an anagram.
'to upset' says the letters should be written in reverse.
'nat' written backwards gives 'tan'.
'it' anagrammed gives 'ti'.
'ti'+'tan'='TITAN'
(Other definitions for titan that I've seen before include "One of the mythological Greek giants who ruled the world" , "heavyweight?" , "Goddess" , "Rhea, perhaps" , "Mythological giant" .)