Nick beset by vermin, getting germ (7)
I believe the answer is:
microbe
'germ' is the definition.
('microbe' can be a synonym of 'germ')
'nick beset by vermin' is the wordplay.
'nick' becomes 'rob' (I've seen this in other clues).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'vermin' becomes 'mice'.
'rob' placed inside 'mice' is 'MICROBE'.
'getting' is the link.
(Other definitions for microbe that I've seen before include "Bacillus" , "source of disease?" , "minor cause for complaint" , "virus" , "Organism causing disease or fermentation" .)