The brief thoughtless beat (6)
I believe the answer is:
thrash
'beat' is the definition.
(synonyms)
'the brief thoughtless' is the wordplay.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'thoughtless' becomes 'rash' (rash can mean reckless or not thought through).
'the' with its final letter removed is 'th'.
'th'+'rash'='THRASH'
(Other definitions for thrash that I've seen before include "Whip repeatedly" , "Drub" , "Trounce, beat soundly" , "tan" , "Flog; party" .)