Racing expert a lot of rubbish? That's mostly harsh (7)
I believe the answer is:
tipster
'racing expert' is the definition.
'tipster' can be an answer for 'expert' (tipster is a kind of expert). I am not sure about the 'racing' bit.
'a lot of rubbish? that's mostly harsh' is the wordplay.
'a lot of rubbish?' becomes 'tip' (I can't explain this - if you can you should believe this answer much more).
'that's mostly' means to remove the last letter (most but not all of the word).
'harsh' becomes 'stern' (I've seen this in another clue).
'stern' with its last letter removed is 'ster'.
'tip'+'ster'='TIPSTER'
Can you help me to learn more?
(Other definitions for tipster that I've seen before include "Adviser on betting" , "Giver of betting advice" , "better counsellor" , "he recommends risky investments" , "One who sells betting advice" .)