Lister worked to keep one germ-free (7)
I believe the answer is:
sterile
'one germ-free' is the definition.
(I've seen this before)
'lister worked to keep' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'lister' is 'steril' which is present in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sterile that I've seen before include "Free from bacteria" , "Uninspired" , "Barren" , "Unable to produce young" , "Non-productive" .)