Colour of ephemeral, deciduous leaves outside (7)
I believe the answer is:
emerald
'colour' is the definition.
(emerald is a kind of colour)
'ephemeral deciduous leaves outside' is the wordplay.
'outside' says the answer is hidden in the clue (a word is around or outside the answer).
'EMERALD' is hidden in the letters of 'ephemeral deciduous leaves'.
'of' is the link.
(Other definitions for emerald that I've seen before include "and 1 Across: Ireland" , "Precious gemstone" , "Green gem" , "Green like this isle" , "Small size of type, in printing" .)