Restriction is on irregular rest in handrails (9)

'handrails' is the definition.
('banister' can be a synonym of 'handrail')

'restriction is on irregular rest' is the wordplay.
'restriction' becomes 'ban' (a ban is a restriction on something).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'irregular' indicates anagramming the letters.
'rest' is an anagram of 'ster'.
'is' going around 'ster' is 'isters'.
'ban'+'isters'='BANISTERS'

'in' is the link.