Nile ibis drifts across middle of river unnoticed (9)
I believe the answer is:
invisible
'unnoticed' is the definition.
(thesaurus)
'nile ibis drifts across middle of river' is the wordplay.
'drifts' is an anagram indicator.
'across' is an insertion indicator.
'middle of' means to look at the middle letters.
The centre of 'river' is 'v'.
'nile'+'ibis'='nileibis'
'nileibis' anagrammed gives 'inisible'.
'inisible' placed around 'v' is 'INVISIBLE'.
(Other definitions for invisible that I've seen before include "Not able to be seen" , "Imperceptible by the eye" , "Blind" , "Writer of unseen words?" , "Out of sight" .)