Head honcho with a large, blue vein perhaps (3,6)
I believe the answer is:
big cheese
'head honcho' is the definition.
(I've seen this in another clue)
'with a large blue vein perhaps' is the wordplay.
'with' means one lot of letters go next to another.
'a large' becomes 'big' (I've seen this before).
'blue vein perhaps' becomes 'cheese' (I can't explain this - if you can you should give a lot more credence to this answer).
'big' put next to 'cheese' is 'BIG CHEESE'.
Can you help me to learn more?
(Other definitions for big cheese that I've seen before include "boss" , "Large cake of Cheshire, perhaps" , "VIP" , "Notable" , "Important person (slang)" .)