Last of the first in first, in a first of the last (5)
I believe the answer is:
final
'last' is the definition.
(I know that last can be written as final)
'the first in first in a first of the last' is the wordplay.
'the first in first' becomes 'f' (1st letter of 'first').
'first of' indicates taking the first letters.
The first letter of 'last' is 'l'.
'f'+'in'+'a'+'l'='FINAL'
'of' acts as a link.
(Other definitions for final that I've seen before include "Last in a series of games or contests" , "Last match in a competition" , "Not to be altered" , "big match" , "Last, ultimate" .)