Musician's very beastly utterance about officer (5)
I believe the answer is:
molto
'musician's very' is the definition.
(Italian term used in musical directions)
'beastly utterance about officer' is the wordplay.
'beastly utterance' becomes 'moo' (sound made by a cow).
'about' means one lot of letters goes inside another.
'officer' becomes 'lt' (abbreviation for lieutenant).
'moo' enclosing 'lt' is 'MOLTO'.
(Other definitions for molto that I've seen before include "Very musical Italian" , "very familiar to players" .)