Birds giving a brownish grey fantail a nasty sock! (8)
I believe the answer is:
dunnocks
'birds' is the definition.
(dunnock is a kind of bird)
'giving a brownish grey fantail a nasty sock' is the wordplay.
'giving' means one lot of letters go next to another.
'a brownish grey fantail' becomes 'dunn' (I can't explain this - if you can you should believe this answer much more).
'a nasty' is an anagram indicator.
'sock' with letters rearranged gives 'ocks'.
'dunn' next to 'ocks' is 'DUNNOCKS'.
Can you help me to learn more?
(Another definition for dunnocks that I've seen is " Hedge sparrows".)