At first patient leaves with fit of shivering — it could be deadly (6)
I believe the answer is:
plague
'it could be deadly' is the definition.
The definition suggests a singular noun which matches the answer.
'at first patient leaves with fit of shivering' is the wordplay.
'at first' says to take the initial letters.
'with' means one lot of letters go next to another.
'fit of shivering' becomes 'ague' (I've seen this before).
The first letters of 'patient leaves' is 'pl'.
'pl'+'ague'='PLAGUE'
Can you help me to learn more?
(Other definitions for plague that I've seen before include "Afflict" , "Lethal epidemic" , "Deadly contagious disease, or a swarm of locusts" , "Badger" , "large scale affliction" .)