A matter to be discussed (2,5)

I believe the answer is:

at issue

'to be discussed' is the definition.

'a matter' is the wordplay.
'matter' becomes 'tissue' (tissue is a kind of matter).
'a'+'tissue'='AT ISSUE'

(Other definitions for at issue that I've seen before include "under debate" , "in dispute" , "Questionable" , "Being discussed or disputed" , "In question, under discussion" .)

I've seen this clue in The Telegraph.

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