Copse's last two trees? Nonsense! (7)
I believe the answer is:
eyewash
'nonsense' is the definition.
(I've seen this before)
'copse's last two trees?' is the wordplay.
'two trees?' can mean two substitutions for 'tree'.
'last' says to take the final letters.
'tree' becomes 'yew' (type of tree).
'tree' becomes 'ash' (ash is a kind of tree).
The final letter of 'copse' is 'e'.
'e'+'yew'+'ash'='EYEWASH'
(Other definitions for eyewash that I've seen before include "Fiddlesticks" , "Pupils may benefit from this" , "Nonsense; cleansing lotion" , "Balderdash" , "Optic solution to relieve irritation - rubbish!" .)