Crossword Genius

Also forming part of the supplement (2,8)

Ross

I believe the answer is:

in addition

'also' is the definition.
('in addition' can be a synonym of 'also')

'part of the supplement' is the wordplay.
'part of' becomes 'in' (I've seen this in other clues).
'the supplement' becomes 'addition' (I've seen this before).
'in'+'addition'='IN ADDITION'

'forming' is the link.

(Another definition for in addition that I've seen is " Also".)

I've seen this clue in the Evening Standard.
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