Naturally obstreperous northern boatmen (2,3,6,4)
I believe the answer is:
to the manner born
'naturally' is the definition.
(I've seen this before)
'obstreperous northern boatmen' is the wordplay.
'obstreperous' indicates anagramming the letters.
'northern'+'boatmen'='northernboatmen'
'northernboatmen' with letters rearranged gives 'TO THE MANNER BORN'.
(Other definitions for to the manner born that I've seen before include "like a natural" , "naturally at ease when working" , "with innate skill" , "Naturally fitted for role" , "with a natural aptitude?" .)