Pipe, not without content that’s free-flowing (6)
I believe the answer is:
fluent
'free-flowing' is the definition.
(I've seen this before)
'pipe not without content' is the wordplay.
'pipe' becomes 'flue' (flue pipe is a kind of pipe).
'without content' means to remove the middle letters.
'not' with its centre removed is 'nt'.
'flue'+'nt'='FLUENT'
'that's' is the link.
(Other definitions for fluent that I've seen before include "Expert in tongue" , "Easy, rapid (speech)" , "Easy with words" , "Speaking with facility" , "Ready in speech" .)