Puzzle that might be draining (6)
I believe the answer is:
riddle
'puzzle' is the definition.
(riddling is a kind of puzzling)
'might be draining' is the wordplay.
'might' becomes 'ridd' (I am not sure about this - if you are sure you should believe this answer much more).
'be' becomes 'lie' (lying is a kind of being).
'draining' suggests removing the centre (I've seen 'drained' mean this).
'lie' with its centre removed is 'le'.
'ridd'+'le'='RIDDLE'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for riddle that I've seen before include "Question to test ingenuity, something puzzling" , "Eg, When is a door not a door?" , "Cryptic question" , "Puzzling person, thing" , "Puzzle or strainer" .)