Big boss mostly crazy to have backed corporation first (3,6)
I believe the answer is:
top banana
'big boss' is the definition.
(I've seen this in another clue)
'have backed corporation first' is the wordplay.
'have' becomes 'banana' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'backed' says the letters should be written backwards.
'corporation' becomes 'pot' (corporation can humorously mean a pot belly).
'first' means one lot of letters go next to another.
'pot' written backwards gives 'top'.
'banana' after 'top' is 'TOP BANANA'.
'mostly crazy to' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for top banana that I've seen before include "pick of the bunch?" , "Big cheese" , "figurehead?" , "organisation chief" , "first on hand?" .)