Romantic sir, I'd fancy, could be racist maybe? (13)
I believe the answer is:
discriminator
'be racist maybe?' is the definition.
(racism is a type of discrimination)
'romantic sir i'd fancy' is the wordplay.
'fancy' indicates anagramming the letters.
'romantic'+'sir'+'id'='romanticsirid'
'romanticsirid' with letters rearranged gives 'DISCRIMINATOR'.
'could' acts as a link.
(Other definitions for discriminator that I've seen before include "DOCTRINAIRISM (anag.)" , "A distinguishing characteristic" .)