Fortuitous, a Parisian camping on Scottish island by loch (13)
I believe the answer is:
unintentional
'fortuitous' is the definition.
(fortuitous can mean not done intentionally)
'a parisian camping on scottish island by loch' is the wordplay.
'a parisian' becomes 'un' ('a' in French).
'camping' becomes 'in tent' (staying in a tent).
'on' is a charade indicator (letters next to each other).
'scottish island' becomes 'iona' (island of the Hebrides).
'by loch' becomes 'l' (I am not sure about the 'by' bit.).
'un'+'intent'+'iona'+'l'='UNINTENTIONAL'
(Other definitions for unintentional that I've seen before include "Talent in union (anag) - not planned" , "Not meant" , "Without much thought" , "Not done on purpose" , "Inadvertent" .)