Unit expresses surprise with onset of march (3)
I believe the answer is:
ohm
'unit' is the definition.
(ohm is a kind of unit)
'surprise with onset of march' is the wordplay.
'surprise with' becomes 'oh' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'onset of' suggests taking the first letters.
The initial letter of 'march' is 'm'.
'oh'+'m'='OHM'
'expresses' is the link.
Can you help me to learn more?
(Other definitions for ohm that I've seen before include "physical unit" , "A little resistance" , "lawmaker" , "Unit of electrical resistance (3)" , "Physicist" .)