By chance (12)
'by chance' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for accidentally that I've seen before include "Not purposefully, by mistake" , "Inadvertently, by chance" .)
'by chance' is the definition.
(I've seen this before)
This is all the clue.
(Another definition for fortuitously that I've seen is " By chance".)
(Other definitions for forte that I've seen before include "speciality" , "Musically, loud" , "Strong point - loudly (in music)" , "Loud (mus.)" , "Personal strong suit" .)