The great man; look it up with Nat, perhaps (5)
I believe the answer is:
titan
'the great man' is the definition.
(I've seen this before)
'it up with nat perhaps' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'with' means one lot of letters go next to another.
'perhaps' indicates an anagram.
'it' in reverse letter order is 'ti'.
'nat' is an anagram of 'tan'.
'ti'+'tan'='TITAN'
'look' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for titan that I've seen before include "A giant; Saturn moon" , "Huge character" , "Person of heroic stature" , "Saturn's largest satellite" , "Person of great strength - moon of Saturn" .)