Diving bird never settled by the ageing GBS? (9)
I believe the answer is:
razorbill
'diving bird' is the definition.
'razorbill' can be an answer for 'bird' (razorbill is a kind of bird). I am unsure of the 'diving' bit.
I don't understand the rest of the clue.
(Other definitions for razorbill that I've seen before include "Type of auk" , "flier" , "Bird" .)