Gain an advantage over nervous 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)
'nervous marshal at once' is the wordplay.
'nervous' indicates anagramming the letters (shaking).
'marshal'+'at'+'once'='marshalatonce'
'marshalatonce' anagrammed gives 'STEAL A MARCH ON'.
(Other definitions for steal a march on that I've seen before include "Slyly gain an advantage over (someone)" , "Anticipate" , "Get ahead of (someone) by taking early action" , "Get the better of" , "Gain early advantage over" .)