Old Lady Walker seen around in processing unit (9)
I believe the answer is:
mainframe
'processing unit' is the definition.
(part of a computer)
'old lady walker seen around in' is the wordplay.
'old lady' becomes 'ma' (both can mean mother).
'walker' becomes 'frame' (walking frame).
'seen around' indicates putting letters inside.
'ma'+'frame'='maframe'
'maframe' placed around 'in' is 'MAINFRAME'.
(Other definitions for mainframe that I've seen before include "Computer's central processing unit" , "powerful computer" , "Large central computer" , "Computer system" , "Mar famine (anag.)" .)