Small number keeping mostly fixed diet (4,6)

I believe the answer is:

lose weight

I'm a little stuck... Click here to teach me more about this clue!

'diet' is the definition.
(thesaurus)

'small number keeping mostly fixed' is the wordplay.
I cannot really see how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
'number' could be 'eight' (eight is a kind of number) and 'eight' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Independent.

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