Greatest resistance, say, put up in final (7)
I believe the answer is:
largest
'greatest' is the definition.
('large' can be a synonym of 'great')
'resistance say put up in final' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'say' becomes 'eg' (both can mean for example).
'put up' says the letters should be written in reverse.
'in' indicates putting letters inside.
'final' becomes 'last' (synonyms).
'eg' backwards is 'ge'.
'r'+'ge'='rge'
'rge' going into 'last' is 'LARGEST'.
(Other definitions for largest that I've seen before include "Maximum" , "Greatest in size" , "Of greatest magnitude" , "most great" , "Most ample" .)