Start of refrain - one very loud repeated phrase (4)
I believe the answer is:
riff
'repeated phrase' is the definition.
(I've seen this in another clue)
'start of refrain one very loud' is the wordplay.
'start of' suggests taking the first letters.
'one' becomes 'i' (Roman numeral).
'very loud' becomes 'ff' (musical abbreviation).
The first letter of 'refrain' is 'r'.
'r'+'i'+'ff'='RIFF'
(Other definitions for riff that I've seen before include "Short repeated jazz figure" , "Short series of chords in jazz" , "Short sequence of musical chords" , "Short repeated phrase in popular music or jazz" , "repeating phrase" .)