Knocks more than once right before starting a proper smash (4)
I believe the answer is:
raps
'knocks' is the definition.
('rap' can be a synonym of 'knock')
'more than once right before starting a proper smash' is the wordplay.
'more than once' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'right' becomes 'r' (common abbreviation).
'before' says to put letters next to each other.
'starting' indicates taking the first letters.
The first letters of 'proper smash' is 'ps'.
'a' put after 'r' is 'ra'.
'ra'+'ps'='RAPS'
Can you help me to learn more?
(Other definitions for raps that I've seen before include "'Knocks, on the door say (4)'" , "Cracks" , "Sprechgesang pop hits" , "Knocks sharply" .)