Copper turning up in helmet, that’s clear (5)
I believe the answer is:
'clear' is the definition.
(similar in meaning)
'copper turning up in helmet' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'turning up' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'helmet' becomes 'lid' (slang for crash helmet).
'cu' reversed gives 'uc'.
'uc' inserted within 'lid' is 'LUCID'.
'that's' is the link.
(Other definitions for lucid that I've seen before include "Easy to understand, clearly expressed" , "Bright; sane" , "Clear, easy to understand" , "Transparent" , "Clear-cut" .)