Aussie screecher giving girl a headache initially (5)
I believe the answer is:
galah
'aussie screecher' is the definition.
(Australian bird which screeches)
'giving girl a headache initially' is the wordplay.
'giving' means one lot of letters go next to another.
'girl' becomes 'gala' (I can't explain this - if you can you should believe this answer much more).
'initially' suggests taking the first letters.
The initial letter of 'headache' is 'h'.
'gala'+'h' is 'GALAH'.
(Other definitions for galah that I've seen before include "Fool" , "Simpleton" .)