Learned about body firmness at group spa treatment (5,7)

'learned about body' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'firmness at group spa treatment' is the wordplay.
I cannot really see how this works, but
'firmness' could be 'tone' (I have seen 'Bodily firmness' mean 'tone' so perhaps 'firmness' could also mean 'tone') and 'tone' is found within the answer.
'treatment' could be 'massage' (I have seen 'therapeutic treatment' mean 'massage' so perhaps 'treatment' could also mean 'massage') and 'massage' is found in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?