Brutal, gendarme initially, running in car thief (3-7)

'thief' is the definition.
(cat burglar is a kind of thief)

'brutal gendarme initially running in car' is the wordplay.
'initially' indicates taking the first letters.
'running' indicates an anagram.
'in' is an insertion indicator.
The first letter of 'gendarme' is 'g'.
'brutal'+'g'='brutalg'
'brutalg' with letters rearranged gives 'atburgl'.
'atburgl' placed into 'car' is 'CAT-BURGLAR'.