Bird erring badly when diving into water (9)
I believe the answer is:
peregrine
'bird' is the definition.
(peregrine is a kind of bird)
'erring badly when diving into water' is the wordplay.
'badly' is an anagram indicator.
'when diving into' is an insertion indicator.
'water' becomes 'pee' (synonyms).
'erring' with letters rearranged gives 'regrin'.
'regrin' put inside 'pee' is 'PEREGRINE'.
(Other definitions for peregrine that I've seen before include "Kind of falcon" , "bird" , "Large falcon" , "Powerful falcon" .)