Stress about identification friend produces by mistake (12)
I believe the answer is:
accidentally
'by mistake' is the definition.
(I've seen this before)
'stress about identification friend' is the wordplay.
'stress' becomes 'accent' (synonyms).
'about' means one lot of letters goes inside another.
'identification' becomes 'ID'.
'friend' becomes 'ally' (synonyms).
'accent' placed around 'id' is 'accident'.
'accident'+'ally'='ACCIDENTALLY'
'produces' acts as a link.
(Other definitions for accidentally that I've seen before include "Inadvertently, by chance" , "Not purposefully, by mistake" .)