Young man has new terrain (4)
I believe the answer is:
land
'terrain' is the definition.
(I've seen this before)
'young man has new' is the wordplay.
'young man' becomes 'lad' (lad can mean a boy or young man).
'has' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'lad' enclosing 'n' is 'LAND'.
(Other definitions for land that I've seen before include "people" , "(Of aircraft) alight" , "See 3" , "Acquire" , "Capture" .)