Fan club ultimately let down? (6)
I believe the answer is:
blower
'fan' is the definition.
(blower is a kind of fan)
'club ultimately let down?' is the wordplay.
'ultimately' indicates one should take the final letters.
'let down?' becomes 'lower' ('lower' can be a synonym of 'let down').
The last letter of 'club' is 'b'.
'b'+'lower'='BLOWER'
(Other definitions for blower that I've seen before include "Speaking-tube" , "Fan; telephone (colloq.)" , "Telephone (informal)" , "It produces an air blast" , "Fan - phone" .)