Craftsman on the fiddle a dull one (5)
I believe the answer is:
amati
'craftsman on the fiddle' is the definition.
'amati' can be an answer for 'fiddle' (violin created by the Amati family). I'm not sure about the 'craftsman on the' bit.
'a dull one' is the wordplay.
'dull' becomes 'mat' (mat as opposed to glossy).
'one' becomes 'i' (Roman numeral).
'a'+'mat'+'i'='AMATI'
Can you help me to learn more?
(Other definitions for amati that I've seen before include "Italian family of violin-makers" , "Noted Cremona violinmaking family" , "fine cello" , "fiddling Italian family" , "fiddler" .)