Old writer, expert the French regarded as not worth keeping on (10)
I believe the answer is:
expendable
'not worth keeping on' is the definition.
I can't judge whether this definition defines the answer.
'old writer expert the french' is the wordplay.
'old' becomes 'ex'.
'writer' becomes 'pen' (I've seen this before).
'expert' becomes 'dab' (I've seen this before).
'the french' becomes 'le'.
'ex'+'pen'+'dab'+'le'='EXPENDABLE'
'regarded as' is the link.
Can you help me to learn more?
(Other definitions for expendable that I've seen before include "Able to be sacrificed" , "Easily sacrificed" , "Easily abandoned" , "not important" .)