Ultimately, he saw the deal — Jack is first, possibly a diamond (5)
I believe the answer is:
jewel
'a diamond' is the definition.
(thesaurus)
'ultimately he saw the deal jack is first' is the wordplay.
'ultimately' says to take the final letters.
'jack' becomes 'J' (playing card abbreviation).
'is first' says to put letters next to each other.
The final letters of 'he saw the deal' are 'ewel'.
'ewel' after 'j' is 'JEWEL'.
'possibly' acts as a link.
(Other definitions for jewel that I've seen before include "eg Ruby" , "Precious stone or mineral" , "Gem" , "The - That Was Ours (Dexter)" , "Gemstone" .)