Junior guy, one mixed up with Resistance (7)
I believe the answer is:
younger
'junior' is the definition.
(the junior of two people is the younger person)
'guy one mixed up with resistance' is the wordplay.
'mixed up with' indicates anagramming the letters.
'resistance' becomes 'r' (electronics abbreviation).
'guy'+'one'='guyone'
'guyone' anagrammed with 'r' makes 'YOUNGER'.
(Other definitions for younger that I've seen before include "More immature" , "More youthful" , "Less grey" , "Less mature" , "more boyish" .)