Eccentric, if excellent corporation (8)
I believe the answer is:
crackpot
'eccentric' is the definition.
(crackpot is a kind of eccentric)
'excellent corporation' is the wordplay.
'excellent' becomes 'crack' (the most excellent, as in a 'crack team').
'corporation' becomes 'pot' (corporation can humorously mean a pot belly).
'crack'+'pot'='CRACKPOT'
'if' acts as a link.
(Other definitions for crackpot that I've seen before include "Mad" , "Idiot" , "Ludicrous" , "Nutty" , "crazy type" .)