No idea why, but I’m at the scene (5,3,2,5)
I believe the answer is:
youve got me there
'no idea why' is the definition.
(phrase said when one doesn't know the answer)
'i'm at the scene' is the wordplay.
I cannot really see how this works, but
'i'm' could be 'me' (both refer to the person speaking or writing) and 'me' is found in the answer.
'at' could be 'her' (I've seen this in other clues) and 'her' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This explanation may well be incorrect...
'but' acts as a link.
(Other definitions for youve got me there that I've seen before include "what to say to taxi-driver on arrival" , "what the fare said on reaching destination?" , "thanks for the lift" , "I haven't the foggiest" , "what the passenger eventually said" .)