Thoroughly beat them, without 'em being careless (6)
I believe the answer is:
thrash
'thoroughly beat' is the definition.
'thrash' can be an answer for 'beat' (I've seen this before). I'm unsure of the 'thoroughly' bit.
'them without 'em being careless' is the wordplay.
'without' is a deletion indicator.
'being careless' becomes 'rash' (I've seen this before).
'them' with 'em' removed is 'th'.
'th'+'rash'='THRASH'
Can you help me to learn more?
(Other definitions for thrash that I've seen before include "Soundly defeat; party" , "Beat violently" , "Flog; party" , "tan" , "Trounce, beat soundly" .)