Dog takes one over a forest (5)
I believe the answer is:
taiga
'forest' is the definition.
(I've seen this before)
'dog takes one over a' is the wordplay.
'dog' becomes 'tag' (synonyms).
'takes' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'over' means one lot of letters go next to another.
'tag' placed around 'i' is 'taig'.
'taig'+'a'='TAIGA'
(Other definitions for taiga that I've seen before include "Swampy Siberian forest" , "Arctic coniferous region" , "Swampy forest south of tundra" , "northern forest" , "Northern region" .)