Machine gun originally seen in middle of Red Square (6)
I believe the answer is:
engine
'machine' is the definition.
(engine is a kind of machine)
'gun originally seen in middle of red square' is the wordplay.
'originally' suggests taking the first letters.
'seen in' indicates putting letters inside.
'middle of' says to take the centre.
'square' becomes 'nine' (I've seen this before**).
The centre of 'red' is 'e'.
The first letter of 'gun' is 'g'.
'e'+'nine'='enine'
'g' inserted into 'enine' is 'ENGINE'.
(Other definitions for engine that I've seen before include "Machine with moving parts" , "Locomotive" , "Machine supplying power" , "Machine converting energy into motion" , "Motor" .)