Firm, lean sort of fighting man (8)
I believe the answer is:
rifleman
'fighting man' is the definition.
'firm lean sort' is the wordplay.
'sort' indicates an anagram.
'firm'+'lean'='firmlean'
'firmlean' with letters rearranged gives 'RIFLEMAN'.
'of' is the link.
(Other definitions for rifleman that I've seen before include "Sniper" , "Soldier carrying gun" , "British private soldier - New Zealand songbird" , "Bird" , "Soldier - crack shot?" .)