Most harsh-sounding notices in shop (8)
I believe the answer is:
raspiest
'most harsh' is the definition.
('raspy' can be a synonym of 'harsh')
'sounding notices in shop' is the wordplay.
'sounding notices' becomes 'spies' (I've seen this in another clue. I am not sure about the 'sounding' bit.).
'in' means one lot of letters goes inside another.
'shop' becomes 'rat' (both can mean to betray someone).
'spies' going inside 'rat' is 'RASPIEST'.
Can you help me to learn more?
(Another definition for raspiest that I've seen is " most harsh noises".)