Base installing British machine (5)
I believe the answer is:
robot
'machine' is the definition.
(I've seen this before)
'base installing british' is the wordplay.
'base' becomes 'root' (synonyms).
'installing' means one lot of letters goes inside another (inserted letters are installed).
'british' becomes 'b' (abbreviation e.g. in BBC).
'root' going around 'b' is 'ROBOT'.
(Other definitions for robot that I've seen before include "Automaton or sci-fi creature" , "Computer-driven entity" , "Automation" , "Unpaid worker" , "programmed servant" .)