The smallest note – a fifty (7)
I believe the answer is:
minimal
'the smallest' is the definition.
(I've seen this before)
'note a fifty' is the wordplay.
'note' becomes 'minim' (minim is a kind of note**).
'fifty' becomes 'l' ('L' can be a synonym of 'fifty').
'minim'+'a'+'l'='MINIMAL'
(Other definitions for minimal that I've seen before include "Very small, the least possible (7)" , "Very little" , "barely worth considering" , "Least possible; negligible" , "Token" .)