New pine trees around London lake (10)
I believe the answer is:
serpentine
'london lake' is the definition.
(I've seen this before)
'new pine trees around' is the wordplay.
'new' becomes 'n'.
'around' indicates anagramming the letters.
'n'+'pine'+'trees'='npinetrees'
'npinetrees' with letters rearranged gives 'SERPENTINE'.
(Other definitions for serpentine that I've seen before include "Devious" , "Meandering" , "water created in London" , "London lake" , "London water feature" .)