Coming across fashionable and well-informed with no backtracking (9,4)

'coming across' is the definition.
('happen upon' can be a synonym of 'come across')

'fashionable and well-informed with no backtracking' is the wordplay.
I cannot quite see how this works, but
'fashionable' could be 'in' (in can mean fashionable or trendy) and 'in' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'informed' could be 'upon' (similar in meaning) and 'upon' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?