Northern flier, good inside, is one who works out how to get there (9)
I believe the answer is:
navigator
'one who works out how to get there' is the definition.
(a navigator works out how to get somewhere)
'northern flier good inside' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'flier' becomes 'aviator' (synonyms).
'good' becomes 'g' (abbreviation).
'inside' indicates putting letters inside.
'aviator' placed around 'g' is 'avigator'.
'n'+'avigator'='NAVIGATOR'
'is' acts as a link.
(Other definitions for navigator that I've seen before include "Prince Henry, say" , "reliable guide" , "Co-driver" , "course director" , "Marine explorer" .)