Sheep climbing over another for grass (6)
I believe the answer is:
marram
'grass' is the definition.
(I've seen this before)
'sheep climbing over another' is the wordplay.
'sheep' becomes 'ram' (male sheep).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'over another' becomes 'ram' (I can't explain this - if you can you should give a lot more credence to this answer).
'ram' back-to-front is 'mar'.
'mar'+'ram'='MARRAM'
'for' acts as a link.
(Other definitions for marram that I've seen before include "Grass of the seaside" , "Sea grass" , "Coarse grass on sandy beaches" .)