Unusual dictionary missing “Y” in index (9)
I believe the answer is:
indicator
'index' is the definition.
(I know that index can be written as indicator)
'unusual dictionary missing y' is the wordplay.
'unusual' indicates anagramming the letters.
'missing' indicates named letters should be taken away.
'dictionary' with 'y' taken out is 'dictionar'.
'dictionar' with letters rearranged gives 'INDICATOR'.
'in' acts as a link.
(Other definitions for indicator that I've seen before include "Station information board" , "Gauge, meter" , "Gauge; information board" , "Flashing car light" , "This shows" .)