Patrol old mills for a lot of money (3,6)
I believe the answer is:
top dollar
'a lot of money' is the definition.
(top dollar means a high price)
'patrol old mills' is the wordplay.
'mills' indicates an anagram (letters mill or swarm around).
'patrol'+'old'='patrolold'
'patrolold' anagrammed gives 'TOP DOLLAR'.
'for' acts as a link.
(Other definitions for top dollar that I've seen before include "high price?" , "High(est) price" , "The highest sum paid" .)