Finish hosting mass dance (5)
I believe the answer is:
stomp
'dance' is the definition.
(stomp is a kind of dance)
'finish hosting mass' is the wordplay.
'finish' becomes 'stop' (synonyms).
'hosting' means one lot of letters goes inside another (inserted letters are welcomed in or hosted).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'stop' enclosing 'm' is 'STOMP'.
(Other definitions for stomp that I've seen before include "tramp" , "Tread very heavily" , "Walk heavily" , "Fast rhythmic dance" , "Tread angrily" .)