Singer, poor thing, mostly grates (8)
I believe the answer is:
nightjar
'singer' is the definition.
(nightjar is a kind of birdanimal)
'poor thing mostly grates' is the wordplay.
'poor' indicates anagramming the letters.
'mostly' means to remove the last letter (most but not all of the word).
'grates' becomes 'jars' (I've seen this in another clue).
'jars' with its last letter taken away is 'jar'.
'thing' is an anagram of 'night'.
'night'+'jar'='NIGHTJAR'
(Other definitions for nightjar that I've seen before include "Noct. bird (of harsh cry)" , "Winger" , "Nocturnal bird with harsh cry" , "Nocturnal bird of swift family" , "Nocturnal bird related to the swift" .)