Fighting resistance where RAF can't, sadly (6,7)
I believe the answer is:
trench warfare
'fighting' is the definition.
(type of warfare or fighting)
'resistance where raf can't sadly' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'sadly' indicates an anagram.
'r'+'where'+'raf'+'cant'='rwhererafcant'
'rwhererafcant' is an anagram of 'TRENCH WARFARE'.
(Other definitions for trench warfare that I've seen before include "combat in the field" , "Combat on Western Front in WW1" , "Combat from dug-in positions" .)