Spotted flyer in extremely tough attack (6)
I believe the answer is:
thrush
'spotted flyer' is the definition.
(bird with spots)
'extremely tough attack' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'attack' becomes 'rush' (rushing is a kind of attacking).
'tough' with its centre taken out is 'th'.
'th'+'rush'='THRUSH'
'in' is the link.
(Other definitions for thrush that I've seen before include "Infection of the mouth and throat" , "Speckled songbird or throat infection" , "Common British songbird" , "Flyer" , "Mouth fungus or songbird" .)