Device ultimately put in so that resistance varied? (8)
I believe the answer is:
rheostat
'device ultimately put in so that resistance varied?' is the definition.
'rheostat' can be an answer for 'device' (rheostat is a kind of device). I am unsure of the remainder of the definition.
'device ultimately put in so that resistance varied?' is the wordplay.
'ultimately' indicates one should take the final letters.
'put in' is an insertion indicator.
'resistance' becomes 'r' (abbreviation).
'varied?' indicates an anagram (I've seen 'varied' mean this).
The final letter of 'device' is 'e'.
'so'+'that'+'r'='sothatr'
'sothatr' is an anagram of 'rhostat'.
'e' inserted inside 'rhostat' is 'RHEOSTAT'.
(Other definitions for rheostat that I've seen before include "Variable resistor" , "showing some resistance" , "Means of controlling resistance" , "Current controller" , "Sort heat with Instrument that controls the current in an electrical circuit" .)