Ran away from Caroline producing a bad strain (1,4)
I believe the answer is:
e coli
'a bad strain' is the definition.
The definition suggests a singular noun which matches the answer.
'ran away from caroline producing' is the wordplay.
'ran away' becomes 'ran' ('run' can be a synonym of 'run away').
'from' indicates named letters should be taken away.
'producing' is an anagram indicator.
'caroline' with 'ran' removed is 'colie'.
'colie' anagrammed gives 'E COLI'.
Can you help me to learn more?
(Other definitions for e coli that I've seen before include "Nasty bug" , "Cause of internal problem" , "cause of serious complaint" , "cause of ill-feeling" , "Type of bacteria causing food poisoning" .)