Nearly completely over bird's defences (6)
I believe the answer is:
alibis
'defences' is the definition.
(alibi is a kind of defence)
'nearly completely over bird's' is the wordplay.
'nearly' means to remove the last letter.
'completely' becomes 'all' ('all' can be a synonym of 'completely').
'over bird' becomes 'ibis' (ibis is a kind of bird. I am not sure about the 'over' bit.**).
'all' with its final letter taken away is 'al'.
'al'+'ibis'='ALIBIS'
Can you help me to learn more?
(Other definitions for alibis that I've seen before include "Proofs of absence from the scene of the crime" , "Defences in court" , "Pleas of being elsewhere at the time of a crime" , "Defences - was elsewhere at time of crimes" , "Excuses" .)