Share nothing, being mad (6)
I believe the answer is:
whacko
'mad' is the definition.
(I've seen this before)
'share nothing' is the wordplay.
'share' becomes 'whack' (whack can mean a share or contribution).
'nothing' becomes 'O' (O resembles 0).
'whack'+'o'='WHACKO'
'being' acts as a link.
(Other definitions for whacko that I've seen before include "nuts" , "Jolly good" , "Mad" , "Eccentric" .)