Man, around 51, is very loud (5)
I believe the answer is:
cliff
'man' is the definition.
(I've seen this before)
'around 51 is very loud' is the wordplay.
'around' becomes 'c' (abbreviation for 'circa').
'51' becomes 'LI' (roman numeral).
'is very loud' becomes 'ff' (musical abbreviation).
'c'+'li'+'ff'='CLIFF'
(Other definitions for cliff that I've seen before include "A sheer rock-face" , "Sir - - Richard" , "Man" , "A steep rock-face on coast" , "stony face?" .)