Copper, on lake trip abroad, finds criminal (7)
I believe the answer is:
culprit
'criminal' is the definition.
(thesaurus)
'copper on lake trip abroad' is the wordplay.
'copper' becomes 'cu' (chemical symbol**).
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'lake' becomes 'l'.
'abroad' indicates an anagram.
'trip' anagrammed gives 'prit'.
'l' put inside 'prit' is 'lprit'.
'cu'+'lprit'='CULPRIT'
'finds' is the link.
(Other definitions for culprit that I've seen before include "One perpetrating crime" , "one awaiting trial" , "Criminal" , "Perpetrator of a wrongdoing" , "The one responsible for a crime" .)