Sign to hold mass with more than one host (6)
I believe the answer is:
armies
'more than one host' is the definition.
(I've seen this in another clue)
'sign to hold mass' is the wordplay.
'sign' becomes 'aries' (star sign).
'to hold' indicates putting letters inside.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'aries' going around 'm' is 'ARMIES'.
'with' acts as a link.
(Other definitions for armies that I've seen before include "Bodies of people organised for war" , "Hosts" , "Organizations of military land forces" , "Large groups of soldiers" , "Multitudes" .)