Relaxed, having one day clear (6)
I believe the answer is:
limpid
'clear' is the definition.
(similar in meaning)
'relaxed having one day' is the wordplay.
'relaxed' becomes 'limp'.
'having' means one lot of letters go next to another.
'one' becomes 'I' (Roman numeral).
'day' becomes 'd' (abbreviation for day).
'limp'+'i'+'d'='LIMPID'
(Other definitions for limpid that I've seen before include "Clear - tranquil" , "Calm" , "Crystal clear like pure water" , "Translucent" , "Completely clear" .)