Pipe conservation group makes smooth (6)
I believe the answer is:
fluent
'smooth' is the definition.
(I know that smooth can be written as fluent)
'pipe conservation group' is the wordplay.
'pipe' becomes 'flue' (flue is a kind of pipe).
'conservation group' becomes 'NT' (abbreviation for National Trust).
'flue'+'nt'='FLUENT'
'makes' is the link.
(Other definitions for fluent that I've seen before include "Able to express oneself easily" , "Competent (of language) - graceful (of movement)" , "Free-flowing" , "Easily able to speak and write" , "Easy, rapid (speech)" .)