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 subsidiary indication.
'nervous' indicates an anagram (shaking).
'marshalatonce' with letters rearranged gives 'stealamarchon'.
(Other definitions for steal a march on that I've seen before include "Slyly gain an advantage over (someone)", "Gain early advantage over", "Get ahead of (someone) by taking early action", "Gain an advantage underhandedly", "Gain an advantage over (someone)" and "Gain advantage over by underhand means".)