Mature pier design (4)
I believe the answer is:
ripe
'mature' is the definition.
(similar in meaning)
'pier design' is the wordplay.
'design' indicates an anagram.
'pier' is an anagram of 'RIPE'.
(Other definitions for ripe that I've seen before include "Right" , "Mature, ready to eat" , "Pier (anag.)" , "Seasoned, fully grown" , "Opportune" .)