Three times the amount of rubbish covering lake (6)
I believe the answer is:
triple
'three times' is the definition.
(I've seen this before)
'the amount of rubbish covering lake' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'amount of rubbish' becomes 'ripe' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'covering' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'ripe' going around 'l' is 'riple'.
't'+'riple'='TRIPLE'
Can you help me to learn more?
(Other definitions for triple that I've seen before include "Three times" , "In three parts" , "make much longer" , "Threefold, treble" , "Consisting of three" .)