Behind seaside structure, more choppy (7)
I believe the answer is:
bumpier
'more choppy' is the definition.
Both the answer and definition are comparative adjectives. Perhaps you can see an association between them that I don't see?
'behind seaside structure' is the wordplay.
'behind' becomes 'bum' (synonyms).
'seaside structure' becomes 'pier' (I've seen this before).
'bum'+'pier'='BUMPIER'
Can you help me to learn more?
(Other definitions for bumpier that I've seen before include "rougher" , "less smooth" .)