Derive by reasoning (6)
I believe the answer is:
deduce
'derive by reasoning' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for deduce that I've seen before include "Arrive at conclusion through logic" , "Infer like Sherlock" , "Reason out (something)" , "Arrive at reasoned conclusion from facts" , "Work out" .)