Reserve ultimate in opulent beach houses (3,5)
I believe the answer is:
set aside
'reserve' is the definition.
(synonyms)
'ultimate in opulent beach houses' is the wordplay.
'ultimate in' says to take the final letters.
'beach' becomes 'seaside' (synonyms).
'houses' means one lot of letters goes inside another.
The final letter of 'opulent' is 't'.
't' put into 'seaside' is 'SET-ASIDE'.
(Other definitions for set aside that I've seen before include "The policy of taking land out of production to reduce crop surplus" , "Preserve" , "Discount" , "kept for later" , "Reserve for future use" .)