Spoil dumped on lake is clayey rock (4)
I believe the answer is:
marl
'clayey rock' is the definition.
'marl' can be an answer for 'rock' (I've seen this before). I'm unsure of the 'clayey' bit.
'spoil dumped on lake' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'dumped on' means one lot of letters go next to another.
'lake' becomes 'L' (geographical abbreviation).
'mar'+'l'='MARL'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for marl that I've seen before include "Clayey fertiliser" , "Clayey rock or soil" , "Type of clay used as fertilizer" , "Clay/silt fertiliser" .)