Solver seen in agony — ouch! (3)
I believe the answer is:
you
'solver' is the definition.
(the person solving the clue!)
'in agony ouch' is the wordplay.
'in' says the answer is hidden in the clue.
'YOU' can be found hidden inside 'agony ouch'.
'seen' is the link.
(Other definitions for you that I've seen before include "Person being addressed" , "Second-person pronoun" , "The audience as addressed by speaker" , "' - - and Yours' (R4)" , "Second person pronoun" .)