Cross, Times pursues copper stifling resistance (4)
I believe the answer is:
crux
'cross' is the definition.
(thesaurus)
'times pursues copper stifling resistance' is the wordplay.
'times' becomes 'x' (x resembles the multiplication or times sign).
'pursues' says to put letters next to each other.
'copper' becomes 'Cu' (chemical symbol).
'stifling' means one lot of letters goes inside another (some letters get stifled or contained).
'resistance' becomes 'r' (electronics abbreviation).
'cu' placed around 'r' is 'cru'.
'x' put after 'cru' is 'CRUX'.
(Other definitions for crux that I've seen before include "Decisive point of an issue" , "puzzle" , "Pivotal point - cross" , "Nub" , "Essence or gist" .)