Irish police in Italian lake (5)
I believe the answer is:
garda
This clue is a double definition.
'irish police' is the first definition.
(member of the Irish police)
'italian lake' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for garda that I've seen before include "Italian lake - Irish police" , "An Irish limb of the law" , "Irish police officer" , "Policeman of the Irish Republic" , "Member of the police" .)