Sober, with saintly assistance (5)

I believe the answer is:

staid

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'sober' is the definition.
(synonyms)

'saintly assistance' is the wordplay.
'saintly' becomes 'st' (I can't explain this - if you can you should give a lot more credence to this answer).
'assistance' becomes 'aid' (synonyms).
'st'+'aid'='STAID'

'with' is the link.

Can you help me to learn more?

(Other definitions for staid that I've seen before include "Grave, sedate" , "Unadventurous, dull" , "Stiff; serious minded" , "Proper; sedate" , "Settled" .)

I've seen this clue in the Evening Standard.

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