Declaration made by a very loud girl to five relations (9)
I believe the answer is:
affidavit
'declaration' is the definition.
(I have seen 'Legal declaration' mean 'affidavit' so perhaps 'declaration' could also mean 'affidavit')
'a very loud girl to five relations' is the wordplay.
'very loud' becomes 'ff' (musical abbreviation).
'girl' becomes 'ida' ().
'to five' becomes 'V' (Roman numeral).
'relations' becomes 'it' (both can mean sex).
'a'+'ff'+'ida'+'v'+'it'='AFFIDAVIT'
'made by' is the link.
(Other definitions for affidavit that I've seen before include "Legal declaration" , "Sworn statement for legal purposes" , "Instrument's" , "legal document" , "Written declaration made under oath" .)