Great reptile with different head (6)
I believe the answer is:
wizard
'great' is the definition.
(I've seen this before)
'with different head' is the wordplay.
'with' becomes 'w' (short for with).
'different head' becomes 'izard' (I can't explain this - if you can you should believe this answer much more).
'w'+'izard'='WIZARD'
'reptile' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for wizard that I've seen before include "One who practices magic or sorcery" , "Jolly good" , "Magician, necromancer" , "guide to PC" , "Genius" .)