Ply trade primarily with unsophisticated local head (9)
I believe the answer is:
thickness
'ply' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'trade primarily with unsophisticated local head' is the wordplay.
'primarily' says to take the initial letters.
'with' is a charade indicator (letters next to each other).
'unsophisticated' becomes 'hick' (synonyms).
'local head' becomes 'ness' (both can mean a promontory. I am not sure about the 'local' bit.).
The initial letter of 'trade' is 't'.
't'+'hick'+'ness'='THICKNESS'
Can you help me to learn more?
(Other definitions for thickness that I've seen before include "Dimension from front to back" , "Diameter" , "Width" , "Breadth" , "Distance through an object" .)