Snub extremely rude fan (6)
I believe the answer is:
rebuff
'snub' is the definition.
(synonyms)
'extremely rude fan' is the wordplay.
'extremely' means to remove the middle letters.
'fan' becomes 'buff' (synonyms).
'rude' with its middle taken out is 're'.
're'+'buff'='REBUFF'
(Other definitions for rebuff that I've seen before include "Spurn, snub" , "Cold-shoulder" , "Blunt rejection" , "Reject in an ungrateful manner" , "Abrupt or ungracious rejection" .)