Impractical idea I dismissed (3,2)
I believe the answer is:
not on
'impractical' is the definition.
(not on can mean impractical or inappropriate)
'idea i dismissed' is the wordplay.
'idea' becomes 'notion' (notion is a kind of idea).
'dismissed' is a deletion indicator.
'notion' with 'i' removed is 'NOT ON'.
(Other definitions for not on that I've seen before include "Out of the question" , "Informally, cannot be permitted or tolerated" , "Informally, can't be accepted or tolerated" , "Definitely inadvisable" , "Modern phrase for unacceptable" .)