Indication of cardinal significance (7,5)
I believe the answer is:
talking point
'indication of cardinal significance' is the definition.
'talking point' can be an answer for 'significance' (talking point is a kind of significance). I am not certain of the 'indication of cardinal' bit.
'cardinal significance' is the wordplay.
'cardinal' becomes 'talking' (I can't justify this - if you can you should give a lot more credence to this answer).
'significance' becomes 'point' (point is a kind of significance).
'talking'+'point'='TALKING-POINT'
Can you help me to learn more?
(Other definitions for talking point that I've seen before include "Topic inviting discussion" , "matter for debate" , "Subject for discussion" , "Matter of discussion" .)