If I’m hired afresh to bring in unionisation at first, it may improve the atmosphere (10)

'it may improve the atmosphere' is the definition.
The definition suggests a singular noun which matches the answer.

'if i'm hired afresh to bring in unionisation at first' is the wordplay.
'afresh' indicates anagramming the letters.
'to bring in' means one lot of letters goes inside another.
'at first' says to take the initial letters.
The initial letter of 'unionisation' is 'u'.
'if'+'im'+'hired'='ifimhired'
'ifimhired' is an anagram of 'hmidifier'.
'hmidifier' going around 'u' is 'HUMIDIFIER'.

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