Somehow stay in good health mentally (6)

I believe the answer is:

sanity

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'good health mentally' is the definition.
I can't tell whether this definition defines the answer.

'somehow stay in' is the wordplay.
'somehow' is an anagram indicator.
'stay'+'in'='stayin'
'stayin' is an anagram of 'SANITY'.

Can you help me to learn more?

(Other definitions for sanity that I've seen before include "presence of mind?" , "Reason and coherence" , "Rational behaviour" , "State of having a healthy mind" , "Rational condition" .)

I've seen this clue in The Irish Independent.

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