Lake beside a port, time to get in! (7)
I believe the answer is:
ontario
'lake' is the definition.
(I've seen this before)
'beside a port time to get in' is the wordplay.
'beside' becomes 'on' ().
'port' becomes 'rio' (Rio de Janeiro is a port).
'time' becomes 't' (abbreviation).
'to get in' means one lot of letters goes inside another.
'on'+'a'+'rio'='onario'
'onario' enclosing 't' is 'ONTARIO'.
(Other definitions for ontario that I've seen before include "Province of central Canada" , "Smallest of the Great Lakes" , "part of N America" , "Canadian Great lakes province" , "Canadian province and Great Lake" .)