They labour, these writers about love (5)

I believe the answer is:

peons

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'they labour these' is the definition.
The definition suggests an adverb but the answer is not.

'writers about love' is the wordplay.
'writers' becomes 'pens' (I've seen this before).
'about' is an insertion indicator.
'love' becomes 'o' (love means zero in tennis).
'pens' enclosing 'o' is 'PEONS'.

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