Firm resistance: read all about it (6)
I believe the answer is:
sturdy
'firm' is the definition.
(firm can mean sturdy or solid)
'resistance read all about it' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'read' becomes 'study' (synonyms).
'all about it' is an insertion indicator (some letters go about or around others).
'r' put inside 'study' is 'STURDY'.
(Other definitions for sturdy that I've seen before include "Stout, brawny" , "Strong, solid" , "Strongly and solidly built" , "Firm" , "Robust, strong" .)