Quiet Yankee eats fast enough (6)
I believe the answer is:
plenty
'enough' is the definition.
(I know that enough can be written as plenty)
'quiet yankee eats fast' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'eats' is an insertion indicator.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'p'+'y'='py'
'py' enclosing 'lent' is 'PLENTY'.
(Other definitions for plenty that I've seen before include "Many" , "More than enough" , "A full supply - lots" , "Copiousness" , "A lot" .)