Fantastic reptile with head becoming wide (6)
I believe the answer is:
wizard
'fantastic' is the definition.
(I've seen this before)
'with head becoming wide' is the wordplay.
'head' indicates taking the first letters.
'becoming wide' becomes 'izard' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'with' is 'w'.
'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 "software guide" , "Ripping" , "Hogwarts' graduate - Super!" , "One who can work wonders" , "Hotshot" .)