... by ignoring conclusions of best idea (6)
I believe the answer is:
beside
'by' is the definition.
(I've seen this in another clue)
'ignoring conclusions of best idea' is the wordplay.
I cannot quite understand how this works, but
'idea' could be 'side' (side is a kind of idea) and 'side' is found within the answer.
The remaining letters 'be' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for beside that I've seen before include "Next to - compared with" , "Next door to" , "- - the point" , "Adjacent to" , "Neighbours" .)