Left university to join detectives, clear? (5)
I believe the answer is:
lucid
'clear?' is the definition.
(similar in meaning)
'left university to join detectives' is the wordplay.
'left' becomes 'l' (common abbreviation).
'university' becomes 'u' (abbreviation for university).
'to join' means one lot of letters go next to another.
'detectives' becomes 'cid' (abbreviation for Criminal Investigation Department).
'l'+'u'+'cid'='LUCID'
(Other definitions for lucid that I've seen before include "completely clear" , "Transparent" , "Clear, sane" , "Sensible" , "Cogent" .)