A single Venetian magistrate drops round, making you nervous (2,4)
I believe the answer is:
on edge
'nervous' is the definition.
(on edge can mean nervous or tense)
'a single venetian magistrate drops round' is the wordplay.
'a single' becomes 'one' (a single thing - only one).
'venetian magistrate' becomes 'doge' (historical term for the chief magistrate of Venice).
'drops' indicates named letters should be taken away.
'round' becomes 'o' (o is a circular letter).
'doge' with 'o' removed is 'dge'.
'one'+'dge'='ON EDGE'
'making you' is the link.
(Other definitions for on edge that I've seen before include "Tense and irritable, with nerves stretched" , "stressed" , "Nervy, tense and irritable" , "Highly strung" , "Cliff-top walker may be" .)