Foremost of experts consulted on lingering infection source (1,4)
I believe the answer is:
e coli
'source' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's a link between them I don't understand?
'foremost of experts consulted on lingering infection' is the wordplay.
'foremost of' says to take the initial letters.
The first letters of 'experts consulted on lingering infection' is 'E COLI'.
(Other definitions for e coli that I've seen before include "Bug that causes food poisoning" , "Some types of this bacterium are toxic" , "cause of illness" , "Type of bacteria causing food poisoning" , "Common species of bacterium" .)