The thing about bargain is coming from wishful thinker (8)
I believe the answer is:
idealist
'wishful thinker' is the definition.
(I've seen this before)
'the thing about bargain is' is the wordplay.
'the thing' becomes 'it'.
'about' is an insertion indicator.
'bargain' becomes 'deal' ('deal' can be a synonym of 'bargain').
'deal'+'is'='dealis'
'it' placed around 'dealis' is 'IDEALIST'.
'coming from' is the link.
(Other definitions for idealist that I've seen before include "Person who acts from the highest motivations" , "One sees things as they should be rather than how they are" , "Romantic sort" , "Visionary" , "One pursuing perfection" .)