Shelved beach contains tidewater, initially (3,5)
I believe the answer is:
set aside
'shelved' is the definition.
(shelve can mean to defer or set aside)
'beach contains tidewater initially' is the wordplay.
'beach' becomes 'sea-side' (synonyms).
'contains' means one lot of letters goes inside another.
'initially' indicates taking the first letters.
The first letter of 'tidewater' is 't'.
'seaside' enclosing 't' 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 surpluses" , "agricultural policy" , "Farm land taken out of production" , "EU's agricultural scheme" , "Policy for leaving land alone" .)