Loud roar coming from fifty entering underneath (6)
I believe the answer is:
bellow
'loud roar' is the definition.
(a loud, deep noise)
'fifty entering underneath' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'entering' is an insertion indicator.
'underneath' becomes 'below' (I've seen this before).
'l' inserted inside 'below' is 'BELLOW'.
'coming from' acts as a link.
(Other definitions for bellow that I've seen before include "Roar, shout" , "Loud and deep roar" , "Loud cry of a bull" , "Shout; Saul --, novelist" , "Deep roar as of bull" .)