Organise a resistance line (7)
I believe the answer is:
arrange
'organise' is the definition.
(arranging is a kind of organizing)
'a resistance line' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'line' becomes 'range' (I've seen this before).
'a'+'r'+'range'='ARRANGE'
(Other definitions for arrange that I've seen before include "Put in order - organise" , "Set in order" , "position" , "Fix" , "Marshal" .)