Bugs in bear and cat, I fancy (8)
I believe the answer is:
bacteria
'bugs' is the definition.
(I've seen this before)
'bear and cat i fancy' is the wordplay.
'and' is a charade indicator (letters next to each other).
'fancy' is an anagram indicator.
'cat'+'i'='cati'
'bear' put after 'cati' is 'catibear'.
'catibear' with letters rearranged gives 'BACTERIA'.
'in' acts as a link.
(Other definitions for bacteria that I've seen before include "Possible cause of disease" , "Care a bit about these micro-organisms" , "I eat crab (anag)" , "Tiny causes of disease" , "Microscopic organisms, may cause disease" .)