Broke off engagement on Facebook? It wasn't meant to be! (10)
I believe the answer is:
unintended
'wasn't meant to be' is the definition.
(not meant or intended)
'broke off engagement on facebook? it' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'broke off' could be 'ended' (I've seen this in another clue) and 'ended' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for unintended that I've seen before include "Not meant" , "Not planned" , "Not deliberate" , "by mistake" .)