Probably best guy to cover electromagnetism, you’re welcome! (2,2,5)
I believe the answer is:
be my guest
'cover electromagnetism you're welcome' is the definition.
I can't judge whether this definition defines the answer.
'probably best guy' is the wordplay.
I cannot really understand how this works, but
an anagram of 'guy' is 'ygu' which is within the answer.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for be my guest that I've seen before include "as 1 [HOTELIERS] might say" , "Do as you wish!" , "Please do!" , "Go ahead" , "Help yourself!" .)