Colour of old grass? (6)
I believe the answer is:
orange
'colour' is the definition.
(orange is a kind of colour)
'old grass?' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'grass?' becomes 'range' (grassing is a kind of ranging).
'o'+'range'='ORANGE'
'of' acts as a link.
(Other definitions for orange that I've seen before include "Order colour" , "An ogre (anag.)" , "network" , "Onager (anag.)" , "Go earn the fruit" .)