Property of trees in meadows: healthy inside with sun (9)
I believe the answer is:
leafiness
'property of trees' is the definition.
'meadows healthy inside with sun' is the wordplay.
'meadows' becomes 'leas' (I've seen this before).
'healthy' becomes 'fine'.
'inside' means one lot of letters goes inside another.
'with' means one lot of letters go next to another.
'sun' becomes 's'.
'leas' enclosing 'fine' is 'leafines'.
'leafines'+'s'='LEAFINESS'
'in' is the link.
(Other definitions for leafiness that I've seen before include "Forest will uphold such" , "green growth" .)