Lice moving around old bug (1,4)
I believe the answer is:
e coli
'bug' is the definition.
(as in E coli bacteria)
'lice moving around old' is the wordplay.
'moving' indicates anagramming the letters (letters are moved around).
'around' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lice' with letters rearranged gives 'ecli'.
'ecli' placed around 'o' is 'E COLI'.
(Other definitions for e coli that I've seen before include "Cause of internal problem" , "cause of ill-feeling" , "Gut bugs" , "cause of illness" , "cause of serious complaint" .)