Loud noise that's heard principally below (7)
I believe the answer is:
thunder
'loud noise' is the definition.
(thesaurus)
'that's heard principally below' is the wordplay.
'principally' says to take the initial letters.
'below' becomes 'under' ('under' can be a synonym of 'below'**).
The first letters of 'that heard' is 'th'.
'th'+'under'='THUNDER'
(Other definitions for thunder that I've seen before include "Big noise" , "Stormy peal" , "Reverberate" , "Red hunt for a loud noise" , "Rumble after lightning" .)