Greatest resistance, say, put up in final (7)
I believe the answer is:
largest
'greatest' is the definition.
(synonyms)
'resistance say put up in final' is the wordplay.
'resistance' becomes 'r' (abbreviation in physics and electronics).
'say' becomes 'eg' (both can mean 'for example').
'put up' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'final' becomes 'last' (synonyms).
'eg' in reverse letter order is 'ge'.
'r'+'ge'='rge'
'rge' inserted inside 'last' is 'LARGEST'.
(Other definitions for largest that I've seen before include "Maximum" , "Greatest in size" , "Of greatest magnitude" , "most great" , "Most ample" .)