Nervous, but working with slight advantage (2,4)
I believe the answer is:
on edge
'nervous' is the definition.
(thesaurus)
'working with slight advantage' is the wordplay.
'working' becomes 'on'.
'with' is a charade indicator (letters next to each other).
'slight advantage' becomes 'edge' (I've seen this before).
'on'+'edge'='ON EDGE'
'but' is the link.
(Other definitions for on edge that I've seen before include "Nervous and tense - close to cliff?" , "'Tense, nervous (2,4)'" , "All agog" , "Cliff-top walker may be" , "Twitchy" .)