Something wet falling on to heather from young tree (7)
I believe the answer is:
sapling
'young tree' is the definition.
(I've seen this before)
'something wet falling on to heather' is the wordplay.
'something wet' becomes 'sap'.
'falling on to' is a charade indicator (letters next to each other) (some letters go on top of others).
'heather' becomes 'ling' ('ling' can be a synonym of 'heather').
'sap'+'ling'='SAPLING'
'from' is the link.
(Other definitions for sapling that I've seen before include "Baby tree" , "Little tree" , "Lapsing into a young tree" , "Young slender tree" , "Poetic youth" .)