Iron left on? Thats criminal (5)
I believe the answer is:
felon
'criminal' is the definition.
('felon' can be a synonym of 'criminal')
'iron left on?' is the wordplay.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'left' becomes 'l' (common abbreviation).
'fe'+'l'+'on'='FELON'
'thats' acts as a link.
(Other definitions for felon that I've seen before include "criminal from Texas" , "Law-breaker" , "Has conviction" , "Serious criminal" , "Malefactor" .)