Sinks on the wagon left covered with spots (7)
I believe the answer is:
settles
'sinks' is the definition.
(settling is a kind of sinking)
'on the wagon left covered with spots' is the wordplay.
'on the wagon' becomes 'TT' (short for teetotal).
'left' becomes 'l' (common abbreviation).
'covered with' means one lot of letters goes inside another.
'spots' becomes 'sees' (I've seen this before).
'tt'+'l'='ttl'
'ttl' going into 'sees' is 'SETTLES'.
(Other definitions for settles that I've seen before include "Pays the bill" , "Long wooden benches with backs" , "Becomes established in a place" , "Resolves" , "furniture" .)