Throw gold at Pollux’s brother (6)
I believe the answer is:
castor
'pollux's brother' is the definition.
(I've seen this in another clue)
'throw gold' is the wordplay.
'throw' becomes 'cast' (synonyms).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'cast'+'or'='CASTOR'
'at' acts as a link.
(Other definitions for castor that I've seen before include "Trolley wheel" , "hat" , "Fur of the beaver" , "Oil beloved by parents of old" , "for wheel? Yes and no" .)