Pitman going round wrong purveyor of headgear (8)
I believe the answer is:
milliner
'purveyor of headgear' is the definition.
(person who makes and sells hats)
'pitman going round wrong' is the wordplay.
'pitman' becomes 'miner' (I've seen this before).
'going round' means one lot of letters goes inside another.
'wrong' becomes 'ill'.
'miner' going around 'ill' is 'MILLINER'.
(Other definitions for milliner that I've seen before include "trader" , "Philip Treacy, say" , "Maker and seller of hats" , "Women's hat maker" , "Maker of women's hats" .)