List unusual clue to Aga (9)
I believe the answer is:
catalogue
'list' is the definition.
(I've seen this before)
'unusual clue to aga' is the wordplay.
'unusual' is an anagram indicator.
'clue'+'to'+'aga'='cluetoaga'
'cluetoaga' anagrammed gives 'CATALOGUE'.
(Other definitions for catalogue that I've seen before include "List of items for sale" , "Brochure containing a list of goods for sale" , "From which we order" , "Actual ego involved in list of items or goods" , "Coagulate into kind of directory" .)