I abandoned idea that's impossible (3,2)
I believe the answer is:
not on
'impossible' is the definition.
(I've seen this before)
'i abandoned idea' is the wordplay.
'abandoned' indicates named letters should be taken away.
'idea' becomes 'notion' (notion is a kind of idea).
'notion' with 'i' taken away is 'NOT ON'.
'that's' is the link.
(Other definitions for not on that I've seen before include "Modern phrase for unacceptable" , "Improper - impossible" , "Impractical" , "Informally, cannot be permitted or tolerated" , "Definitely inadvisable" .)