Finally, assailant and comrade agree (5)
I believe the answer is:
tally
'agree' is the definition.
(synonyms)
'finally assailant and comrade' is the wordplay.
'finally' suggests the final letters.
'and' says to put letters next to each other.
'comrade' becomes 'ally' (I've seen this in another clue).
The final letter of 'assailant' is 't'.
't'+'ally'='TALLY'
(Other definitions for tally that I've seen before include "Agree - count" , "Running total" , "Reckon up" , "label" , "Count up; match" .)