Bug in fleece carried by rodents (7)
I believe the answer is:
microbe
'bug' is the definition.
('microbe' can be a synonym of 'bug')
'fleece carried by rodents' is the wordplay.
'fleece' becomes 'rob' ('rob' can be a synonym of 'fleece').
'carried by' means one lot of letters goes inside another.
'rodents' becomes 'mice' (mouse is a kind of rodent).
'rob' placed within 'mice' is 'MICROBE'.
'in' acts as a link.
(Other definitions for microbe that I've seen before include "Bug, germ" , "Eg, bacterium" , "pathogen" , "minor cause for complaint" , "Tiny organism causing disease" .)