Top copper's held up, that's clear (5)
I believe the answer is:
lucid
'clear' is the definition.
(similar in meaning)
'top copper's held up' is the wordplay.
'top' becomes 'luc' (I can't justify this - if you can you should believe this answer much more).
'copper' becomes 'DI' (detective inspector, type of copper).
'held up' says the letters should be written in reverse.
'di' reversed gives 'id'.
'luc'+'id'='LUCID'
'that's' is the link.
Can you help me to learn more?
(Other definitions for lucid that I've seen before include "Clearly presented and easily understood" , "Intellectually bright" , "Thinking clearly" , "unlike 5 [OPAQUE]" , "Transparent" .)