Menageries house mass of flies (5)
I believe the answer is:
zooms
'flies' is the definition.
The answer and definition can be both acts as well as being plural nouns.
Perhaps you can see an association between them that I don't see?
'menageries house mass' is the wordplay.
'menageries' becomes 'zoos' ('zoo' can be a synonym of 'menagerie').
'house' indicates putting letters inside.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'zoos' going around 'm' is 'ZOOMS'.
'of' is the link.
Can you help me to learn more?
(Other definitions for zooms that I've seen before include "Changes focus rapidly (camera)" , "tears" , "Speeds" , "Whizzes along" .)