Old man confining most of rest in open space (5)
I believe the answer is:
plaza
'open space' is the definition.
(I've seen this before)
'old man confining most of rest' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'confining' indicates putting letters inside.
'most of' means to remove the last letter.
'rest' becomes 'laze'.
'laze' with its final letter removed is 'laz'.
'pa' going around 'laz' is 'PLAZA'.
'in' is the link.
(Other definitions for plaza that I've seen before include "Public square - shopping centre" , "Public square, as in Spanish town" , "open space" , "Open urban square or public area" , "Public square for pedestrians" .)