Bit of a word quietly by referee, and football team retreating (6)
I believe the answer is:
prefix
'bit of a word' is the definition.
(word element at the beginning of a word)
'quietly by referee and football team retreating' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'by referee' becomes 'ref' (synonyms. I am not sure about the 'by' bit.).
'and' means one lot of letters go next to another.
'football team' becomes 'XI' (a football team, from Roman numerals for rr).
'retreating' says the letters should be written in reverse.
'xi' back-to-front is 'ix'.
'p'+'ref'+'ix'='PREFIX'
(Other definitions for prefix that I've seen before include "Element added to the beginning of a word" , "Something added to the start of a word - title before a name" , "Word placed before another to change its meaning" , "Meaningful construct attached to the beginning of a word" , "initial qualifier" .)