Rational detectives left union first (5)
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" .)