Petty theft's first followed by sacking (8)
I believe the answer is:
trifling
'petty' is the definition.
(thesaurus)
'theft's first followed by sacking' is the wordplay.
'first' suggests taking the first letters.
'followed by' means one lot of letters go next to another.
'sacking' becomes 'rifling'.
The initial letter of 'thefts' is 't'.
't'+'rifling'='TRIFLING'
(Other definitions for trifling that I've seen before include "Inconsequential" , "Paltry" , "Minor" , "Frivolous" , "Dallying; petty" .)