Welsh barman I treated after onset of cold (8)
I believe the answer is:
cambrian
'welsh' is the definition.
(I've seen this before)
'barman i treated after onset of cold' is the wordplay.
'treated' is an anagram indicator.
'after' says to put letters next to each other.
'onset of' suggests taking the first letters.
The first letter of 'cold' is 'c'.
'barman'+'i'='barmani'
'barmani' anagrammed gives 'ambrian'.
'ambrian' after 'c' is 'CAMBRIAN'.
(Other definitions for cambrian that I've seen before include "Geological period" , "Relating to Wales" , "Characteristic of Wales or its people" , "Welsh" .)