To measure bird population, Galician starts in northwest Spain. (8)
I believe the answer is:
wingspan
'to measure' is the definition.
(wingspan is a kind of measure)
'bird population galician starts in northwest spain' is the wordplay.
I cannot really understand how this works, but
'in' is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for wingspan that I've seen before include "Avian dimension" , "Lateral spread of an aircraft" , "Width of bird or plane" , "fighter's reach, maybe" , "Distance between the extremites of a bird or plane" .)