Pier design, new and mature (5)
I believe the answer is:
ripen
'mature' is the definition.
(to ripen is to mature)
'pier design new' is the wordplay.
'design' is an anagram indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'pier' anagrammed gives 'ripe'.
'ripe'+'n'='RIPEN'
'and' acts as a link.
(Other definitions for ripen that I've seen before include "Make ready" , "Become mature" , "Season" , "Become ready to pick" , "Become ready for eating" .)