Best possible agreement by leader in industry (5)
I believe the answer is:
ideal
'best possible' is the definition.
(I've seen this before)
'agreement by leader in industry' is the wordplay.
'agreement' becomes 'deal' (deal is a kind of agreement**).
'by' says to put letters next to each other.
'leader in' says to take the initial letters.
The initial letter of 'industry' is 'i'.
'deal' after 'i' is 'IDEAL'.
(Other definitions for ideal that I've seen before include "Model of ethical behaviour" , "In her dreams" , "Perfect but not likely to become a reality" , "existing only in imagination" , "Model of perfection" .)