Iron also reduced inside? Not true (5)
I believe the answer is:
false
'not true' is the definition.
('false' is the opposite to 'true')
'iron also reduced inside?' is the wordplay.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'reduced' means to remove the last letter.
'inside?' means one lot of letters goes inside another.
'also' with its last letter taken off is 'als'.
'fe' going around 'als' is 'FALSE'.
(Other definitions for false that I've seen before include "Forged" , "Bogus" , "Incorrect" , "Artificial" , "Unfaithful" .)