Loud noise made by half of them below (7)
I believe the answer is:
thunder
'loud noise' is the definition.
(I've seen this before)
'by half of them below' is the wordplay.
'by' means one lot of letters go next to another.
'half of' suggests halving the letters.
'below' becomes 'under' (synonyms).
'them' halved is 'th'.
'th' next to 'under' is 'THUNDER'.
'made' is the link.
(Other definitions for thunder that I've seen before include "Loud and prolonged noise in the sky" , "Noise named after the god Thor" , "Noise heard after lightning" , "Strong denunciation" , "Heavenly rumbling" .)