An upright transaction is just what's wanted (5)
I believe the answer is:
ideal
'just what's wanted' is the definition.
I can't judge whether this defines the answer.
'an upright transaction' is the wordplay.
'an' becomes 'i' (Roman numeral for one).
'upright transaction' becomes 'deal' (a deal is a transaction or exchange. I am not sure about the 'upright' bit.).
'i'+'deal'='IDEAL'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "To which one aspires" , "Perfect but not likely to become a reality" , "Model of perfection" , "Optimum" , "Conceptual" .)