Nonsense written about old British machine (5)
I believe the answer is:
robot
'machine' is the definition.
(I've seen this before)
'nonsense written about old british' is the wordplay.
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'written about' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'o'+'b'='ob'
'rot' enclosing 'ob' is 'ROBOT'.
(Other definitions for robot that I've seen before include "Mechanical man" , "Automatic worker?" , "Automaton or sci-fi creature" , "performer lacking emotion" , "Humanoid machine" .)