Borrow book? French novel you once observed to be about railway (6,3)
I believe the answer is:
romany rye
'borrow book?' is the definition.
(1857 novel by George Borrow)
'french novel you once observed to be about railway' is the wordplay.
'french novel' becomes 'roman' ('novel' in French).
'you once' becomes 'ye' (historical version of 'you').
'observed to be about' is an insertion indicator (some letters go around or about others).
'railway' becomes 'ry' (abbreviation for railway).
'ye' going around 'ry' is 'yrye'.
'roman'+'yrye'='ROMANY RYE'
(Other definitions for romany rye that I've seen before include "Borrow book" , "Fellow traveller, as it were" , "One having the gypsy spirit" , "LITERATURE" .)