Duke describes worker as a Walter Mitty character (9)
I believe the answer is:
fantasist
'a walter mitty character' is the definition.
(Walter Mitty is a fictional fantasist)
'duke describes worker as' is the wordplay.
'duke' becomes 'fist' (put up your dukes).
'describes' is an insertion indicator (describe can mean to form the outline of).
'worker' becomes 'ant' (worker is a type of ant).
'ant'+'as'='antas'
'fist' placed around 'antas' is 'FANTASIST'.
(Other definitions for fantasist that I've seen before include "Walter Mitty type?" , "Dreamer" , "Creator of very imaginative fiction" .)