Unknown member of ruling party, one making minor contribution (5)

I believe the answer is:

incog

'unknown' is the definition.
(short for incognito)

'member of ruling party one making minor contribution' is the wordplay.
'member of ruling party' becomes 'in' (ie in government rather than out of it).
'one making minor contribution' becomes 'cog' (figuratively, a cog in a machine).
'in'+'cog'='INCOG'

(Another definition for incog that I've seen is " Under an assumed name (abbrev.)".)

I've seen this clue in The Independent.

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