Eccentric chap — Welsh (8)
I believe the answer is:
cambrian
'welsh' is the definition.
(I've seen this before)
'eccentric chap' is the wordplay.
'eccentric' becomes 'cam' (an eccentric is a mechanical device such as a cam).
'chap' becomes 'brian' ().
'cam'+'brian'='CAMBRIAN'
(Other definitions for cambrian that I've seen before include "Relating to Wales" , "Geological period" , "Characteristic of Wales or its people" , "Welsh" .)