Peak current setter goes after (4)
I believe the answer is:
acme
'peak' is the definition.
(synonyms)
'current setter goes after' is the wordplay.
'current' becomes 'ac' (alternating current).
'setter' becomes 'me' (the setter of the clue).
'goes after' means one lot of letters go next to another.
'ac'+'me'='ACME'
(Other definitions for acme that I've seen before include "Summit of achievement" , "Culmination" , "High point of excellence" , "Summit - maximum" , "Zenith - optimum" .)