Rational detectives left union first (5)
I believe the answer is:
lucid
'rational' is the definition.
(similar in meaning)
'detectives left union first' is the wordplay.
'detectives' becomes 'cid' (abbreviation for Criminal Investigation Department).
'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' put after 'lu' is 'LUCID'.
(Other definitions for lucid that I've seen before include "between raves?" , "Clear, intelligible" , "Intellectually bright" , "Coherent" , "Clear, easily understandable" .)