Are massing round one's rear (5)
I believe the answer is:
raise
'rear' is the definition.
(I know that rear is a kind of raise)
'are massing round one's' is the wordplay.
'are massing' becomes 'rae' (I can't justify this - if you can you should believe this answer much more).
'round' is an insertion indicator.
'one's' becomes 'i's' (Roman numeral).
'rae' going around 'is' is 'RAISE'.
Can you help me to learn more?
(Other definitions for raise that I've seen before include "climb" , "better?" , "jack up" , "Breed" , "rally" .)