Concentrated in core of listeners' riddle, say (9)
I believe the answer is:
intensive
'concentrated' is the definition.
(synonyms)
'core of listeners' riddle say' is the wordplay.
'core of' becomes 'inten' (I can't justify this - if you can you should give a lot more credence to this answer).
'listeners'' indicates a 'sounds like' (homophone) clue.
'riddle say' becomes 'sieve' (riddle is a kind of sieve).
'sieve' is a homophone of 'sive'.
'inten'+'sive'='INTENSIVE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for intensive that I've seen before include "Demanding" , "Very vigorous or thorough; concentrating" , "Concentrated" .)