Get public or private key from address with C-lightning
lightningd doesn't offer simple RPC commands to dump keys.
If you really need to access the keys the simplest way would be to derive them directly from the hsm_secret.
Keys for onchain transactions (since you mentioned newaddr. The other derivations are more involved) are derived using BIP32 unhardened derivation from m/0/0.
The root seed is derived from the hsm_secret with :
root_seed = hkdf_sha256(hsm_secret, salt=NULL, info=&quot;bip32 seed&quot;)

There exist tools such as the hsmtools (which currently only derive keys for the commitment transactions but might be extended in the furure) or jb55's clightning-dumpkeys which take care of the derivation for you.