Mature pier design (4)
I believe the answer is:
ripe
'mature' is the definition.
(I know that mature can be written as ripe)
'pier design' is the wordplay.
'design' is an anagram indicator.
'pier' anagrammed gives 'RIPE'.
(Other definitions for ripe that I've seen before include "Right" , "Mature, ready to eat" , "Pier (anag.)" , "Seasoned, fully grown" , "Opportune" .)