Trainee uniform for plain clothes men, that's clear (5)
I believe the answer is:
lucid
'clear' is the definition.
(similar in meaning)
'trainee uniform for plain clothes men' is the wordplay.
'trainee' becomes 'l' (as in an L-plate for a learner driver).
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'for' says to put letters next to each other.
'plain clothes men' becomes 'cid' (I can't justify this - if you can you should give a lot more credence to this answer).
'l'+'u'+'cid'='LUCID'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for lucid that I've seen before include "Clear and comprehensible" , "Clear, intelligible" , "Rational, clear" , "Sensible" , "between raves?" .)