Just ignore source of something unusually rough and loud (5,3)
I believe the answer is:
shrug off
'just ignore' is the definition.
'source of something unusually rough and loud' is the wordplay.
'source of' indicates taking the first letters.
'unusually' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'loud' becomes 'ff' (a musical term).
The initial letter of 'something' is 's'.
'rough' with letters rearranged gives 'hrugo'.
's'+'hrugo'+'ff'='SHRUG OFF'
(Other definitions for shrug off that I've seen before include "Dismiss (something) as unimportant" , "Brush aside, disregard the importance of" , "Get rid of - show indifference to" , "Rid oneself of" .)