Endless force involved in substantial counter-attack (8)
I believe the answer is:
reprisal
'endless force' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'involved in substantial counter-attack' is the wordplay.
'involved in' is an insertion indicator.
'substantial' becomes 'real' (synonyms).
'counter attack' becomes 'pris' (I am not sure about this - if you are sure you should believe this answer much more).
'real' going around 'pris' is 'REPRISAL'.
Can you help me to learn more?
(Other definitions for reprisal that I've seen before include "Retaliatory action against an enemy" , "comeback" , "compensation" , "Act of retaliation or vengeance" , "Counterattack" .)