Europeans heated pool upon mounting second filter (9)
I believe the answer is:
spaniards
'europeans' is the definition.
(people from Spain)
'heated pool upon mounting second filter' is the wordplay.
'heated pool' becomes 'spa' (I can't justify this - if you can you should give a lot more credence to this answer).
'upon mounting' says the letters should be written in reverse (in a down clue, letters mount up).
'second' becomes 's' (common abbreviation).
'filter' becomes 'drain' (I've seen this before).
's'+'drain'='sdrain'
'sdrain' back-to-front is 'niards'.
'spa'+'niards'='SPANIARDS'
Can you help me to learn more?
(Other definitions for spaniards that I've seen before include "Natives of Seville, say" , "Pass dinar to Europeans" , "Iberians" .)