Reduced rust hadn't affected what fencer might do (3,3,6)
I believe the answer is:
cut-and-thrust
'fencer might do' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'reduced rust hadn't affected' is the wordplay.
'reduced' becomes 'cut' (similar in meaning).
'affected' indicates an anagram.
'rust'+'hadnt'='rusthadnt'
'rusthadnt' is an anagram of 'andthrust'.
'cut'+'andthrust'='CUT-AND-THRUST'
'what' is the link.
Can you help me to learn more?
(Other definitions for cut-and-thrust that I've seen before include "Lively debate, fight" , "Lively and competitive environment" , "Spirited exchange of opinions in debate" .)