Pieces composed by partner in particular (10)

I believe the answer is:

especially

'in particular' is the definition.
(synonyms)

'pieces composed by partner' is the wordplay.
'composed' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'partner' becomes 'ally' (I've seen this in other clues).
'pieces' is an anagram of 'especi'.
'especi'+'ally'='ESPECIALLY'

(Another definition for especially that I've seen is " In particular".)

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