Another thing entirely, indeed, involving guerrilla army resistance (1,3,3)
I believe the answer is:
a far cry
'another thing entirely' is the definition.
'indeed involving guerrilla army resistance' is the wordplay.
'indeed' becomes 'ay'.
'involving' indicates putting letters inside.
'guerrilla army' becomes 'farc' (Revolutionary Armed Forces of Colombia).
'resistance' becomes 'r' (abbreviation).
'farc'+'r'='farcr'
'ay' going around 'farcr' is 'A FAR CRY'.
(Other definitions for a far cry that I've seen before include "Yodel" , "another cup of tea" , "That's a very different matter" , "Greatly different, like a distant shout" , "something very different" .)