Diluted (7)
'diluted' is the definition.
('thin' can be a synonym of 'dilute')
This is the entire clue.
(Other definitions for thinned that I've seen before include "Reduced" , "Made less thick" , "Diluted" .)
'diluted' is the definition.
(watering down is a kind of diluting)
This is all the clue.
(Other definitions for watered down that I've seen before include "weak" , "Diluted" , "Less extreme" , "less interesting?" , "no longer neat" .)