Hard barriers, old and new, within seats (5,5)
I believe the answer is:
stone walls
'hard barriers' is the definition.
(stone walls are sturdy barriers)
'old and new within seats' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'and' is a charade indicator (letters next to each other).
'within' indicates putting letters inside.
'seats' becomes 'stalls'.
'o'+'new'='onew'
'onew' inserted into 'stalls' is 'STONE WALLS'.
(Other definitions for stone walls that I've seen before include "boundaries" , "What boring batsman does" .)