Slammer, finally -- bad result for cattle thief (7)
I believe the answer is:
rustler
'cattle thief' is the definition.
('rustler' can be a synonym of 'cattle thief')
'slammer finally bad result' is the wordplay.
'finally' indicates one should take the final letters.
'bad' is an anagram indicator.
The final letter of 'slammer' is 'r'.
'result' is an anagram of 'ustler'.
'r'+'ustler'='RUSTLER'
'for' acts as a link.
(Other definitions for rustler that I've seen before include "He stole cattle in the old west" , "Horse thief" , "Cattle-robber" , "Stealer of livestock" , "Stock taker?" .)