Part of foot is under bottom, reportedly (5)

I believe the answer is:
arsis
'part of foot' is the definition.
'arsis' can be an answer for 'foot' (thesaurus). I am unsure of the 'part of' bit.
'is under bottom reportedly' is the wordplay.
'under' says to put letters next to each other (below, in a down clue).
'bottom reportedly' becomes 'ars' (I can't justify this - if you can you should give a lot more credence to this answer).
'is' put after 'ars' is 'ARSIS'.
Can you help me to learn more?
(Other definitions for arsis that I've seen before include "no longer raising voice" , "Stressed syllable in verse" .)
