Crossword Genius

Rational detectives left union first (5)

Ross

I believe the answer is:

lucid

'rational' is the definition.
('lucid' can be similar in meaning to 'rational')

'detectives left union first' is the wordplay.
'detectives' becomes 'cid' (**).
'left' becomes 'l' (common abbreviation).
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'first' is a charade indicator (letters next to each other).
'l'+'u'='lu'
'cid' after 'lu' is 'LUCID'.

(Other definitions for lucid that I've seen before include "between raves?" , "Clear, intelligible" , "Intellectually bright" , "Coherent" , "Clear, easily understandable" .)

I've seen this clue in The Mirror.
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