Hard to get bird to wail (4)
I believe the answer is:
howl
'to wail' is the definition.
(synonyms)
'hard to get bird' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to get' says to put letters next to each other.
'bird' becomes 'owl' (owl is a kind of bird).
'h'+'owl'='HOWL'
(Other definitions for howl that I've seen before include "Wolf sound" , "Long cry of dog" , "Long wailing cry" , "Lazy person" , "Plaintive wolf cry" .)