Sweet, please be aware: one hardened at the centre (8)
I believe the answer is:
noisette
'sweet' is the definition.
(I've seen this before)
'aware one hardened at the centre' is the wordplay.
'aware' becomes 'note' (I can't justify this - if you can you should give a lot more credence to this answer).
'one' becomes 'i' (Roman numeral).
'hardened' becomes 'set' (synonyms).
'at the centre' indicates putting letters inside.
'i'+'set'='iset'
'note' placed around 'iset' is 'NOISETTE'.
'please be' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for noisette that I've seen before include "Rolled piece of lamb; nutty sweet" , "Small piece of boneless lamb" , "Some rolled meat" , "Hazelnut chocolate" , "Chocolate made with hazelnuts" .)