Fuss in south-east ultimately concerning old sailor (3,3)
I believe the answer is:
sea dog
'old sailor' is the definition.
(I've seen this before)
'fuss in south-east ultimately concerning' is the wordplay.
'fuss' becomes 'ado' ('ado' can be a synonym of 'fuss').
'in' means one lot of letters goes inside another.
'south east' becomes 'se'.
'ultimately' indicates one should take the final letters.
The final letter of 'concerning' is 'g'.
'se'+'g'='seg'
'ado' going into 'seg' is 'SEA-DOG'.
(Other definitions for sea dog that I've seen before include "Dosage (anag)" , "An old sailor of canine origin" , "mythical creature" , "Old tar" , "Experienced sailor (colloq.)" .)