The copper taking a pound to trip up is the guilty one (7)
I believe the answer is:
culprit
'the guilty one' is the definition.
(I've seen this in another clue)
'the copper taking a pound to trip up' is the wordplay.
'the copper' becomes 'cu' (Cu is the chemical symbol for copper).
'taking' says to put letters next to each other.
'a pound' becomes 'L' (the pound sign is based on an L).
'to' says to put letters next to each other.
'up' indicates anagramming the letters (letters get chucked up).
'trip' anagrammed gives 'prit'.
'cu'+'l'+'prit'='CULPRIT'
'is' is the link.
(Other definitions for culprit that I've seen before include "The person to blame, the guilty one" , "Wrongdoer" , "he should have conviction" , "baddie" , "The one to blame, guilty party" .)