Hurry after extremely tough bird (6)
I believe the answer is:
thrush
'bird' is the definition.
(I know that thrush is a type of oscine)
'hurry after extremely tough' is the wordplay.
'hurry' becomes 'rush' (synonyms).
'after' means one lot of letters go next to another.
'extremely' suggests removing the centre (only the extremes of the word are used).
'tough' with its middle removed is 'th'.
'rush' put after 'th' is 'THRUSH'.
(Other definitions for thrush that I've seen before include "Throstle" , "Infection of the mouth and throat" , "Speckle-breasted songbird" , "Brown songbird" , "Songbird - fungal infection" .)