Gold coin unknown as penny farthing (8)
I believe the answer is:
ordinary
'penny farthing' is the definition.
(I know that penny farthing can be written as ordinary)
'gold coin unknown' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'coin' becomes 'dinar' (monetary unit in some countries).
'unknown' becomes 'y' (algebra).
'or'+'dinar'+'y'='ORDINARY'
'as' is the link.
(Other definitions for ordinary that I've seen before include "Usual - bishop in his province" , "pedestrian" , "Run of the mill" , "Regular" , "Not worthy of noting in X-Files" .)