Outlaw to plunder gold, lifting unknown amount (3,3)
I believe the answer is:
rob roy
'outlaw' is the definition.
(Scottish outlaw)
'plunder gold lifting unknown amount' is the wordplay.
'plunder' becomes 'rob' (to plunder is to rob or pillage).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'lifting' says the letters should be written backwards.
'unknown amount' becomes 'y' (algebra).
'or' reversed gives 'ro'.
'rob'+'ro'+'y'='ROB ROY'
'to' is the link.
(Other definitions for rob roy that I've seen before include "Famous 17th century bandit of Scottish highlands" , "Scottish outlaw, subject of a Sir Walter Scott novel" , "book" , "Scottish outlaw and folk hero, d. 1734" , "Persistently trouble" .)