Unpaid assistant sketching thus? (8)
I believe the answer is:
freehand
'sketching thus?' is the definition.
(as in drawing or sketching freehand)
'unpaid assistant' is the wordplay.
'unpaid' becomes 'free' (similar in meaning).
'assistant' becomes 'hand' (hand can mean a worker or employee).
'free'+'hand'='FREEHAND'
(Other definitions for freehand that I've seen before include "Ignoring ruler, perhaps" , "Without drawing aids" , "Drawn naturally" , "drawn in the normal way" , "given to plenipotentiary" .)