Start to peak ahead of time like nacre (6)
I believe the answer is:
pearly
'nacre' is the definition.
(thesaurus)
'start to peak ahead of time' is the wordplay.
'start to' says to take the initial letters.
'ahead of time' becomes 'early' (synonyms).
The first letter of 'peak' is 'p'.
'p'+'early'='PEARLY'
'like' is the link.
(Other definitions for pearly that I've seen before include "These gates are heavenly" , "Bejewelled" , "like a king in East London?" , "Substance of heaven's gates" , "decorated with gems" .)