Substitute for 'taser' may be 'tazer', essentially (6)
I believe the answer is:
ersatz
'substitute' is the definition.
(synonyms)
'taser may be tazer essentially' is the wordplay.
'may be' indicates anagramming the letters.
'essentially' means to look at the middle letters.
The centre of 'tazer' is 'z'.
'taser' is an anagram of 'ersat'.
'ersat'+'z'='ERSATZ'
'for' acts as a link.
(Other definitions for ersatz that I've seen before include "Made in imitation - in Germany" , "Bogus" , "Phoney as in Germany" , "Mock" , "Spurious" .)