Bug over loch is evil (7)
I believe the answer is:
illness
'bug' is the definition.
(thesaurus)
'over loch is evil' is the wordplay.
'over' says to put letters next to each other.
'loch' becomes 'ness' (Loch Ness in Scotland).
'is evil' becomes 'ill' (I've seen this in other clues).
'ness' put after 'ill' is 'ILLNESS'.
(Other definitions for illness that I've seen before include "Period of sickness" , "Ailment, disease" , "suffering this" , "Indisposition, sickness" , "Disease, sickness" .)