Following mass movement, tries terms — as in US (10)
I believe the answer is:
trimesters
'terms as in us' is the definition.
'trimesters' can be an answer for 'terms' (I've seen this before). I'm not sure about the 'as in us' bit.
'following mass movement tries' is the wordplay.
I cannot really understand how this works, but
'mass' could be 'm' (symbol used in Physics e.g. f=ma) and 'm' is found within the answer.
an anagram of 'tries' is 'riest' which is present in the remaining letters.
This explanation may well be incorrect...
(Other definitions for trimesters that I've seen before include "Terms" , "quarters" .)