Living to accommodate mass emotion (9)
I believe the answer is:
sentiment
'emotion' is the definition.
(I've seen this before)
'living to accommodate mass' is the wordplay.
'living' becomes 'sentient' (I can't explain this - if you can you should give a lot more credence to this answer).
'to accommodate' indicates putting letters inside.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'sentient' going around 'm' is 'SENTIMENT'.
Can you help me to learn more?
(Other definitions for sentiment that I've seen before include "Emotional thought" , "Slush" , "judgement" , "Emotion, self-indulgent or insincere perhaps" , "Tender or romantic emotion" .)