Reserve time invested in place for pier (3,5)
I believe the answer is:
set aside
'reserve' is the definition.
(I know that reserve can be written as set aside)
'time invested in place for pier' is the wordplay.
'time' becomes 't' (abbreviation).
'invested in' means one lot of letters goes inside another.
'place for pier' becomes 'seaside'.
't' put into 'seaside' is 'SET-ASIDE'.
(Other definitions for set aside that I've seen before include "Fallow" , "not productive land" , "Lay by" , "Policy for leaving land alone" , "Save or discard" .)