I'm surprised, first to meet a measure of resistance (3)
I believe the answer is:
ohm
'a measure of resistance' is the definition.
(I've seen this before)
'i'm surprised first to meet' is the wordplay.
'i'm surprised' becomes 'oh' (expression of surprise).
'first to' indicates taking the first letters.
The initial letter of 'meet' is 'm'.
'oh'+'m'='OHM'
(Other definitions for ohm that I've seen before include "lawmaker" , "SI unit of electrical resistance" , "measure of resistance" , "physical unit" , "A little resistance" .)