Expert traitor finding answer is proportional (3,4)
I believe the answer is:
pro rata
'proportional' is the definition.
(I've seen this before)
'expert traitor finding answer' is the wordplay.
'expert' becomes 'pro' (short for professional).
'traitor' becomes 'rat' (I've seen this in another clue).
'finding' says to put letters next to each other.
'answer' becomes 'a' (abbreviation eg in Q and A).
'pro'+'rat'+'a'='PRO RATA'
'is' acts as a link.
(Other definitions for pro rata that I've seen before include "Proportionately based" , "Proportionally" , "In proportion (Latin)" , "fairly divided" , "maintaining correspondence" .)