New in - boy's head is wider than it is tall (9)
I believe the answer is:
landscape
'wider than it is tall' is the definition.
I can't judge whether this defines the answer.
'new in boy's head' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'in' means one lot of letters goes inside another.
'boy's' becomes 'lads' (informal term for boy).
'head' becomes 'cape' (both can mean a piece of land jutting over the sea).
'n' going inside 'lads' is 'lands'.
'lands'+'cape'='LANDSCAPE'
'is' is the link.
Can you help me to learn more?
(Other definitions for landscape that I've seen before include "Type of painting" , "Piece of inland scenery" , "scene" , "Picture showing countryside" , "Views" .)