Doctor in commercial clothing trade (8)
I believe the answer is:
plumbing
'trade' is the definition.
(plumbing is a kind of trade)
'doctor in commercial clothing' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'commercial' becomes 'plug' (I've seen this in another clue).
'clothing' means one lot of letters goes inside another (some letters cover up or clothe others).
'mb'+'in'='mbin'
'mbin' going within 'plug' is 'PLUMBING'.
(Other definitions for plumbing that I've seen before include "Testing depth" , "System of pipes" , "Household water pipes" , "Pipework etc" , "Pipes and fixtures dealing with water and sewage" .)