Rotter is gathering people together (6,6)
I believe the answer is:
compos mentis
'together' is the definition.
(thesaurus)
'rotter is gathering people' is the wordplay.
'rotter' becomes 'compost' (I can't justify this - if you can you should give a lot more credence to this answer).
'gathering' means one lot of letters goes inside another.
'people' becomes 'men' (men can refer to humanity as a whole).
'compost'+'is'='compostis'
'compostis' placed around 'men' is 'COMPOS MENTIS'.
Can you help me to learn more?
(Other definitions for compos mentis that I've seen before include "Of sound mind (Latin)" , "Capable of reasoning" , "Sane" .)