Third person about to be found at this location (4)
I believe the answer is:
here
'at this location' is the definition.
(I've seen this in another clue)
'third person about to be' is the wordplay.
'third person' becomes 'r' (erd letter of 'person').
'about' means one lot of letters goes inside another.
'to be' becomes 'hee' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'r' inserted inside 'hee' is 'HERE'.
'found' acts as a link.
Can you help me to learn more?
(Other definitions for here that I've seen before include "In attendance" , "final clue?" , "Present - at this point" , "On this spot" , ""___ be dragons" (old map inscription?)" .)