Keep off the grass and other plants (8)

'keep' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'the grass and other plants' is the wordplay.
'the grass' becomes 'weed' ('weed' can be a synonym of 'grass').
'and' means one lot of letters go next to another.
'other plants' becomes 'duck' (I can't explain this - if you can you should give a lot more credence to this answer).
'weed' put after 'duck' is 'DUCKWEED'.

'off' acts as a link.

Can you help me to learn more?