Damage unknown following on agreement (7)
I believe the answer is:
harmony
'agreement' is the definition.
(I know that harmony is a type of agreement)
'damage unknown following on' is the wordplay.
'damage' becomes 'harm' (synonyms).
'unknown' becomes 'y' (algebra).
'following' says to put letters next to each other.
'y' after 'on' is 'ony'.
'harm'+'ony'='HARMONY'
(Other definitions for harmony that I've seen before include "Sweet or melodious sound, no disagreement" , "Source of universe for Dryden" , "Mellifluous sound" , "sounds agreeable" , "Pleasing combination of musical notes" .)