People who look very sour must change and start to smile (9)

I believe the answer is:

surveyors

'people who look' is the definition.

'very sour must change and start to smile' is the wordplay.
'must change' indicates anagramming the letters (I've seen this in other clues).
'and' says to put letters next to each other.
'start to' indicates taking the first letters.
The initial letter of 'smile' is 's'.
'very'+'sour'='verysour'
'verysour' anagrammed gives 'surveyor'.
'surveyor'+'s'='SURVEYORS'

I've seen this clue in The Independent.

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