Weed remaining on empty lake (6)
I believe the answer is:
nettle
'weed' is the definition.
(nettle is a kind of weed)
'remaining on empty lake' is the wordplay.
'remaining on' becomes 'nett' (I can't explain this - if you can you should give a lot more credence to this answer).
'empty' suggests removing the centre.
'lake' with its middle taken out is 'le'.
'nett'+'le'='NETTLE'
Can you help me to learn more?
(Other definitions for nettle that I've seen before include "it seems to flower only if dead" , "Upset" , "Growing" , "Annoy - stinger" , "Annoy; common weed" .)