Keeps loud sort moving (5)
I believe the answer is:
forts
'keeps' is the definition.
'loud sort moving' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'moving' indicates an anagram (letters are moved around).
'sort' is an anagram of 'orts'.
'f'+'orts'='FORTS'
(Other definitions for forts that I've seen before include "Strongholds" , "Castles" , "Defensive structures where troops are stationed" .)