A robin, er, surprisingly, flying (8)
I believe the answer is:
airborne
'flying' is the definition.
(thesaurus)
'a robin er surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters.
'robin'+'er'='robiner'
'robiner' is an anagram of 'irborne'.
'a'+'irborne'='AIRBORNE'
(Other definitions for airborne that I've seen before include "upwardly mobile" , "in the sky" , "In flight after taking off" , "Having taken off" , "as birds often are" .)