Bird watcher (8)
I believe the answer is:
twitcher
'bird watcher' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for twitcher that I've seen before include "Bird-watcher (colloq)" , "Name for a keen bird-watcher" , "pursuing hobby?" , "Dedicated birdwatcher" , "Bird watcher" .)