Site Celts developed to produce plums (9)

I believe the answer is:

testicles

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'produce plums' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.

'site celts developed' is the wordplay.
'developed' is an anagram indicator.
'site'+'celts'='sitecelts'
'sitecelts' is an anagram of 'TESTICLES'.

'to' acts as a link.

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