Those missing lessons -- five down -- they'll go downhill rapidly (6)
I believe the answer is:
skiers
'those missing lessons five down they'll go downhill rapidly' is the definition.
I know that 'downhill' relates to this answer.
'go downhill rapidly' is the wordplay.
'go downhill' becomes 'ski' (I've seen this before).
'rapidly' becomes 'ers' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ski'+'ers'='SKIERS'
Can you help me to learn more?
(Other definitions for skiers that I've seen before include "Persons on the slopes" , "Winter sportsmen" , "Kisser among snow sportsmen" , "They race on snow" , "Highballs" .)