Duo in ancient wood (3)
I believe the answer is:
two
'duo' is the definition.
(a duo is two people)
'in ancient wood' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TWO' is hidden in the letters of 'ancient wood'.
(Other definitions for two that I've seen before include "Number in company, some say" , "prime -- that's not odd" , "'It takes ? to tango'" , "Minimal number of partners" , "limited company?" .)