Party by coastal structure is comparatively stupid (6)
I believe the answer is:
dopier
'comparatively stupid' is the definition.
('dopey' can be similar in meaning to 'stupid')
'party by coastal structure' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'by' means one lot of letters go next to another.
'coastal structure' becomes 'pier'.
'do'+'pier'='DOPIER'
'is' acts as a link.
(Other definitions for dopier that I've seen before include "More stupid" , "more idiotic?" .)