Gain an advantage over rotten marshal at once (5,1,5,2)
I believe the answer is:
steal a march on
'gain an advantage over' is the definition.
(I've seen this before)
'rotten marshal at once' is the wordplay.
'rotten' indicates an anagram.
'marshal'+'at'+'once'='marshalatonce'
'marshalatonce' is an anagram of 'STEAL A MARCH ON'.
(Other definitions for steal a march on that I've seen before include "Gain early advantage over" , "Anticipate" , "Get the better of" , "Slyly gain an advantage over (someone)" , "Get ahead of (someone) by taking early action" .)