Expert on pipe work (7)
I believe the answer is:
product
'work' is the definition.
(thesaurus)
'expert on pipe' is the wordplay.
'expert' becomes 'pro' (I've seen this before).
'on' means one lot of letters go next to another.
'pipe' becomes 'duct' (**).
'pro'+'duct'='PRODUCT'
(Other definitions for product that I've seen before include "Result of multiplication" , "Commodity offered for sale" , "Manufactured article for sale" , "Drop cut (anag.)" , "Manufactured object" .)