Endless sarcasm offered by good conductor (4)
I believe the answer is:
iron
'good conductor' is the definition.
(iron conducts electricity)
'endless sarcasm' is the wordplay.
'endless' means to remove the last letter.
'sarcasm' becomes 'irony' (synonyms).
'irony' with its last letter taken off is 'IRON'.
'offered by' acts as a link.
(Other definitions for iron that I've seen before include "stern" , "Press - element" , "Main element in steel" , "Hard metal" , "Strong metal; golf club" .)