Dull keys to study (6)
I believe the answer is:
deaden
'dull' is the definition.
(thesaurus)
'keys to study' is the wordplay.
'keys' becomes 'de' (I've seen this in other clues).
'to study' becomes 'aden' (I can't explain this - if you can you should give a lot more credence to this answer).
'de'+'aden'='DEADEN'
Can you help me to learn more?
(Other definitions for deaden that I've seen before include "Make numb, deprive of feeling" , "Check" , "dull" , "Kill" , "Muffle, deprive of feeling" .)