Compound legislature headed by a Republican (8)

I believe the answer is:

arsenate

'compound' is the definition.
(type of chemical compound)

'legislature headed by a republican' is the wordplay.
'legislature' becomes 'senate' (senate is a kind of legislature).
'headed by' says to put letters next to each other.
'republican' becomes 'r' (abbreviation).
'a'+'r'='ar'
'senate' after 'ar' is 'ARSENATE'.

(Another definition for arsenate that I've seen is " salt".)

I've seen this clue in The Guardian.

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