Sentimentality and a dash of dreaminess can be excellent (4)
I believe the answer is:
good
'be excellent' is the definition.
(thesaurus)
'sentimentality and a dash of dreaminess' is the wordplay.
'sentimentality' becomes 'goo' (informal term for sentimentality).
'and' is a charade indicator (letters next to each other).
'a dash of' indicates taking the first letters (a small amount of the word).
The initial letter of 'dreaminess' is 'd'.
'goo'+'d'='GOOD'
'can' is the link.
(Other definitions for good that I've seen before include "Well done! Health-giving" , "promoting welfare" , "benefit" , "Beneficial" , "To be desired; pleasing" .)