Moisture found in barrier across river next to quiet headland (8)
I believe the answer is:
dampness
'moisture' is the definition.
(being damp or moist)
'barrier across river next to quiet headland' is the wordplay.
'barrier across river' becomes 'dam'.
'next to' is a charade indicator (letters next to each other).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'headland' becomes 'ness' (both can mean a promontory).
'dam'+'p'+'ness'='DAMPNESS'
'found in' is the link.
(Other definitions for dampness that I've seen before include "Being a bit wet" , "Moisture" , "Moist quality" .)