Index was oddly missing information (4)
I believe the answer is:
news
'information' is the definition.
(news is a kind of information)
'index was oddly missing' is the wordplay.
'oddly missing' indicates alternate letters.
'index'+'was'='indexwas'
The alternate letters of 'indexwas' are 'NEWS'.
(Other definitions for news that I've seen before include "'No....is good....'" , "Latest information" , "Contents of bulletin" , "Sewn (anag.)" , "Topical information" .)