Idea, if lacking information, is unacceptable (6)
I believe the answer is:
notion
'idea' is the definition.
(I know that idea can be written as notion)
'lacking information is unacceptable' is the wordplay.
I cannot really see how this works, but
'lacking' could be 'no' (eg lacking money means with no money) and 'no' is located in the answer.
'information' could be 'i' (common abbreviation for information) and 'i' is located in the answer.
The remaining letters 'ton' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'if' is the link.
Can you help me to learn more?
(Other definitions for notion that I've seen before include "clue" , "Whim or fancy" , "Whimsical idea or small sewing article" , "A vague and possibly incorrect opinion" , "Idea or whim" .)