Second person starts to yell “Ow!” unnecessarily (3)
I believe the answer is:
you
'second person' is the definition.
(I've seen this before)
'starts to yell ow unnecessarily' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'yell ow unnecessarily' is 'YOU'.
(Other definitions for you that I've seen before include "My solvers in general" , "Thee" , "Second-person pronoun" , "Reader" , "Anyone" .)