I'm having trouble understanding this phrase:
En blasé à qui on ne la fait plus, je réponds: (...)
The (...) is of course the narrator's response to someone else's question.
There are several difficulties in this fragment, let's take them in order.
First, the preposition en in this context means roughly “speaking as …” (or sometimes “acting as”). It is followed by a quality which justifies the rest of the statement.
This use of the preposition en calls for more than just a noun afterwards: en pigeon, en syndicaliste, etc. feel weird to me. I suspect that this is because en has many meanings and with a mere noun the phrase feels ambiguous. En bon + noun is a common combination with this meaning.
This construct usually indicates an actual quality but can also a indicate a posture.
The adjective blasé, which is used here as a noun, means someone who's seen or practiced a lot about a particular topic. It is originally the past participle of the verb blaser. The verb etimologically meant becoming used to drinking a lot of alcohol, and generalized to becoming insensitive to anything through overexposure. The verb has become rare nowadays but the adjective is still common.
The idiom à qui on ne la fait pas means someone who will not get caught by a scam. Literally: “to which one does not make [the joke].”
The variant on ne [me] la fait plus indicates someone who might have fallen for the trick earlier in life, but who is now sufficiently savvy to notice and avoid it.
Thus à qui on ne la fait plus does barely more than reinforce blasé, which is necessary for the use of en blasé ... to be understood.
À qui on ne la fait plus simply means that the speakers is experienced enough to spot a trick of some sort. Isn't there an english saying about old dogs and tricks that can be repurposed?
Le Robert réserve une entrée pour l'adjectif blasé :
Parmi les exemples :
et une citation que l'on pourrait retourner à votre narrateur :