Criminal partner used a credit card loophole, and 1000 police dropped the ball (10)
I believe the answer is:
accomplice
'criminal partner' is the definition.
(I've seen this before)
'loophole and 1000 police dropped the ball' is the wordplay.
'loophole' becomes 'acco' (I can't explain this - if you can you should believe this answer much more).
'and' says to put letters next to each other.
'a000' becomes 'M' (Roman numeral).
'dropped' indicates named letters should be taken away.
'the ball' becomes 'o' (O looks like a ball).
'police' with 'o' taken away is 'plice'.
'acco'+'m'+'plice'='ACCOMPLICE'
'used a credit card' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for accomplice that I've seen before include "ally" , "Colleague" , "criminal partner" , "Criminal's associate" , "Partner in plan" .)