Wild herb covering third of valley (6)
I believe the answer is:
savage
'wild' is the definition.
(similar in meaning)
'herb covering third of valley' is the wordplay.
'herb' becomes 'sage' (sage is a kind of herb).
'covering' means one lot of letters goes inside another.
'third of valley' becomes 'va' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sage' placed around 'va' is 'SAVAGE'.
Can you help me to learn more?
(Other definitions for savage that I've seen before include "Barbaric, uncivilised" , "Ferocious" , "Wild, untamed" , "Cruel; wild" , "Brutally criticise" .)