Conclude by reasoning (5)
I believe the answer is:
infer
'conclude by reasoning' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for infer that I've seen before include "Deduce (from evidence)" , "Conclude or judge from facts" , "Derive as a consequence" , "come to a conclusion" , "Deduce by reasoning" .)