Mad? As mad as Fanny, perhaps (5)
I believe the answer is:
adams
'fanny perhaps' is the definition.
(as in the phrase 'sweet Fanny Adams')
'mad? as mad' is the wordplay.
'mad?' is an anagram indicator (I've seen 'mad' mean this).
'as'+'mad'='asmad'
'asmad' with letters rearranged gives 'ADAMS'.
'as' acts as a link.
(Other definitions for adams that I've seen before include "revolutionary leader" , "Presidential father or son" , "Father and son were American presidents" , "Douglas - (The Hitchhiker's Guide to the Galaxy)" , "Name of two early US presidents" .)