Former book about this French missile (6)
I believe the answer is:
exocet
'missile' is the definition.
(I've seen this before)
'former book about this french' is the wordplay.
'former' becomes 'ex'.
'book' becomes 'ot' (I've seen this in other clues**).
'about' indicates putting letters inside.
'this french' becomes 'ce' (I've seen this in other clues).
'ot' enclosing 'ce' is 'ocet'.
'ex'+'ocet'='EXOCET'
(Other definitions for exocet that I've seen before include "French-built anti-ship missile" , "Short range guided missile" , "Short-range guided missile" , "Tactical missile" , "which may be guided to hit ship" .)