Onslaughts by Cupid perhaps could be fatal (5,7)
I believe the answer is:
heart attacks
'could be fatal' is the definition.
I can't tell whether this definition defines the answer.
'onslaughts by cupid' is the wordplay.
'onslaughts' becomes 'attacks' ('attack' can be a synonym of 'onslaught').
'by' is a charade indicator (letters next to each other).
'cupid' becomes 'heart' (I can't justify this - if you can you should believe this answer much more).
'attacks' after 'heart' is 'HEART ATTACKS'.
'perhaps' acts as a link.
Can you help me to learn more?
(Another definition for heart attacks that I've seen is " Serious coronary episodes".)