Penetrate some barren terrain (5)
I believe the answer is:
enter
'penetrate' is the definition.
(I know that penetrate is a more specific form of the action penetrate)
'some barren terrain' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ENTER' is hidden within 'barren terrain'.
(Other definitions for enter that I've seen before include "Enroll, sign up" , "put in for a race" , "Go in and put in the ledger" , "cross the threshold" , "Move into" .)