Reserve place for pier around the fourth of September (3,5)
I believe the answer is:
set aside
'reserve' is the definition.
('set aside' can be a synonym of 'reserve')
'place for pier around the fourth of september' is the wordplay.
'place for pier' becomes 'seaside'.
'around' means one lot of letters goes inside another.
'the fourth of september' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'seaside' enclosing 't' is 'SET-ASIDE'.
Can you help me to learn more?
(Other definitions for set aside that I've seen before include "Discount" , "agricultural policy" , "Fallow" , "The policy of taking land out of production to reduce crop surpluses" , "Reserve for future use" .)