Come in from green terrain (5)
I believe the answer is:
enter
'come in' is the definition.
(synonyms)
'from green terrain' is the wordplay.
'from' indicates the answer is hidden within the clue.
'ENTER' can be found hidden inside 'green terrain'.
(Other definitions for enter that I've seen before include "A key on the right" , "Take part in a contest" , "Record (in a log)" , "Register formally as a participant" , "Go in - set down" .)