Trio mostly raving originally about sick suspense story (8)
I believe the answer is:
thriller
'suspense story' is the definition.
(I've seen this in another clue)
'trio mostly raving originally about sick' is the wordplay.
'trio' becomes 'three' (synonyms).
'mostly' means to remove the last letter (most but not all of the word).
'raving originally' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'sick' becomes 'ill' (similar in meaning).
'three' with its last letter removed is 'thre'.
'thre'+'r'='threr'
'threr' enclosing 'ill' is 'THRILLER'.
Can you help me to learn more?
(Other definitions for thriller that I've seen before include "Very exciting contest" , "A detective story can be sensational" , "exciting work" , "Book or film designed to keep us in suspense" , "The Birds', say" .)