Mistreated those trapped by informer — that keeps resistance under control (8)
I believe the answer is:
rheostat
'that keeps resistance under control' is the definition.
(device controlling electrical resistance)
'mistreated those trapped by informer' is the wordplay.
'mistreated' indicates an anagram.
'trapped by' is an insertion indicator.
'informer' becomes 'rat' (synonyms).
'those' anagrammed gives 'heost'.
'heost' inserted within 'rat' is 'RHEOSTAT'.
(Other definitions for rheostat that I've seen before include "Sort heat with Instrument that controls the current in an electrical circuit" , "Current-changing device" , "Variable resistor" , "Resistor for regulating current" , "Electrical controller" .)