Guesses details change unit also (12)

I believe the answer is:

speculations

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'guesses' is the definition.
('speculation' can be a synonym of 'guess')

'details change unit also' is the wordplay.
'details' becomes 'spec' (I can't explain this - if you can you should believe this answer much more).
'change' is an anagram indicator.
'unit'+'also'='unitalso'
'unitalso' anagrammed gives 'ulations'.
'spec'+'ulations'='SPECULATIONS'

Can you help me to learn more?

I've seen this clue in The Daily Express.

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