Last bit of resistance of retreating enemy (3)
I believe the answer is:
foe
'enemy' is the definition.
(foe can mean an enemy or rival)
'last bit of resistance of retreating' is the wordplay.
'last bit of' says to take the final letters.
'retreating' says the letters should be written in reverse.
The last letter of 'resistance' is 'e'.
'e'+'of'='eof'
'eof' reversed gives 'FOE'.
(Other definitions for foe that I've seen before include "Evil rival" , "Antagonist" , "Adversary" , "Opposition" , "Am archaic enemy" .)