Top-ranking members are intoxicated then flat (4-5)

I believe the answer is:

high-level

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'top-ranking members are' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs sometimes define each other.

'intoxicated then flat' is the wordplay.
'intoxicated' becomes 'high' (similar in meaning).
'then' is a charade indicator (letters next to each other).
'flat' becomes 'level' (synonyms).
'high'+'level'='HIGH-LEVEL'

Can you help me to learn more?

(Other definitions for high-level that I've seen before include "Involving VIPs" , "(Of, eg, talks) involving important people" .)

I've seen this clue in the Sydney Morning Herald.

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