One preparation of lead is best (5)
I believe the answer is:
ideal
'is best' is the definition.
(similar in meaning)
'one preparation of lead' is the wordplay.
'one' becomes 'i' (Roman numeral).
'preparation of' is an anagram indicator (I've seen this in other clues).
'lead' with letters rearranged gives 'deal'.
'i'+'deal'='IDEAL'
(Other definitions for ideal that I've seen before include "supreme" , "In her dreams" , "Pattern" , "Quite perfect, can't be improved on" , "Just perfect, couldn't be better" .)