Nutty writer embracing a useless cause (5)
I believe the answer is:
pecan
'a useless cause' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'nutty writer embracing' is the wordplay.
'nutty' becomes 'ca' (I am not sure about this - if you are sure you should believe this answer much more).
'writer' becomes 'pen' (something which writes).
'embracing' is an insertion indicator.
'ca' going within 'pen' is 'PECAN'.
Can you help me to learn more?
(Other definitions for pecan that I've seen before include "Nut tree of the southern United States and Mexico" , "Something like a walnut" , "Walnut-like nut" , "US nut tree" , "Type of nut" .)