By proxy according to expert (3,3)
I believe the answer is:
per pro
'by proxy' is the definition.
(I've seen this before)
'according to expert' is the wordplay.
'according to' becomes 'per' (I've seen this before).
'expert' becomes 'pro' (I've seen this before).
'per'+'pro'='PER PRO'
(Other definitions for per pro that I've seen before include "Abbreviation used signing a letter for another" , "How one signs for another" , "By proxy" , "'pp' (signature) in full" .)