A flat for one some singleton's flipped (4)
I believe the answer is:
note
'a flat for one' is the definition.
(A flat is a musical note)
'some singleton's flipped' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'flipped' says the letters should be written backwards.
'eton' can be found hidden inside 'singletons'.
'eton' reversed gives 'NOTE'.
(Other definitions for note that I've seen before include "Observe; short letter" , "Short record or letter" , "Short informal letter" , "Record - piece of music" , "Short written message" .)