Duck, as it happens, a shade of green (5)
I believe the answer is:
olive
'a shade of green' is the definition.
(I've seen this before)
'duck as it happens' is the wordplay.
'duck' becomes 'o'.
'as it happens' becomes 'live' (as in live television).
'o'+'live'='OLIVE'
(Other definitions for olive that I've seen before include "Girl's name - colour" , "Fruit giving oil" , "One [TYPE OF TREE] is from the Mediterranean" , "Small green Mediterranean tree-fruit (5)" , "A Med. fruit" .)