By chance I had to interrupt speech by friend (12)
I believe the answer is:
accidentally
'by chance' is the definition.
(synonyms)
'i had to interrupt speech by friend' is the wordplay.
'i had' becomes 'I'd' (contraction).
'to interrupt' is an insertion indicator.
'speech' becomes 'accent' (accent is a kind of speech).
'by' is a charade indicator (letters next to each other).
'friend' becomes 'ally' (an ally is a friend or supporter).
'id' inserted within 'accent' is 'accident'.
'accident'+'ally'='ACCIDENTALLY'
(Other definitions for accidentally that I've seen before include "Not purposefully, by mistake" , "Inadvertently, by chance" .)