Goad dubious supporters, they're not genuine! (8)
I believe the answer is:
spurious
'not genuine' is the definition.
(I've seen this before)
'goad dubious supporters' is the wordplay.
'goad' becomes 'spur' (synonyms).
'dubious supporters' becomes 'ious' (I can't justify this - if you can you should give a lot more credence to this answer).
'spur'+'ious'='SPURIOUS'
'they're' is the link.
Can you help me to learn more?
(Other definitions for spurious that I've seen before include "Inauthentic" , "Forged" , "fraudulent" , "Plausible, but false" , "deceitful!" .)