Main line in disarray? It's flowing (8)
I believe the answer is:
seamless
'flowing' is the definition.
(flowing without visible transitions)
'main line in disarray?' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' means one lot of letters goes inside another.
'disarray?' becomes 'mess' (something in disarray is in a mess).
'l' inserted within 'mess' is 'mless'.
'sea'+'mless'='SEAMLESS'
'it's' is the link.
(Other definitions for seamless that I've seen before include "Without apparent joins" , "Unbroken" , "Without obvious joins" , "Showing no joins" , "Showing no join" .)