Northern boatmen circulating completely naturally (2,3,6,4)
I believe the answer is:
to the manner born
'naturally' is the definition.
(I've seen this before)
'northern boatmen circulating' is the wordplay.
'circulating' is an anagram indicator.
'northern'+'boatmen'='northernboatmen'
'northernboatmen' with letters rearranged gives 'TO THE MANNER BORN'.
'completely' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for to the manner born that I've seen before include "Naturally fitted for role" , "accustomed to certain standards" , "with a natural aptitude?" , "with innate skill" , "naturally at ease when working" .)