The last, not the first — great (4)
I believe the answer is:
mega
'great' is the definition.
(I've seen this before)
'the last not the first' is the wordplay.
'the last' becomes 'omega' (I've seen this before).
'not the first' means to remove the first letter.
'omega' with its initial letter taken off is 'MEGA'.
(Other definitions for mega that I've seen before include "Enormous; very good" , "Game (anag) -- great" , "Fantastic!" , "Terrific" , "As brilliant" .)