Loud little boy turned modest (6)
I believe the answer is:
flashy
'loud' is the definition.
('flashy' can be a synonym of 'loud')
'little boy turned modest' is the wordplay.
'little boy' becomes 'alf' (shortened version of Alfred or Alfie**).
'turned' says the letters should be written backwards.
'modest' becomes 'shy'.
'alf' back-to-front is 'fla'.
'fla'+'shy'='FLASHY'
(Other definitions for flashy that I've seen before include "Cheap and showy" , "Superficially brilliant" , "Showily stylish" , "Tastelessly showy" , "Gaudy or showy" .)