Gain an advantage over nervous marshal at once (5,1,5,2)
I believe the answer is:
steal a march on
Here is my best explanation:
'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).
'marshalatonce' anagrammed gives 'STEAL A MARCH ON'.
(Other definitions for steal a march on that I've seen before include "Anticipate", "Gain advantage over by underhand means", "Get ahead of (someone) by taking early action", "Get the better of", "Gain early advantage over", "Gain an advantage underhandedly", "Gain an advantage over (someone)" and "Slyly gain an advantage over (someone)".)