Careful to keep in the final beat (5)
I believe the answer is:
weary
'beat' is the definition.
(thesaurus)
'careful to keep in the final' is the wordplay.
'careful' becomes 'wary' ('wary' can be similar in meaning to 'careful').
'to keep in' indicates putting letters inside.
'final' says to take the final letters.
The last letter of 'the' is 'e'.
'wary' going around 'e' is 'WEARY'.
(Other definitions for weary that I've seen before include "Enervated" , "Worn-out" , "flag" , "Flagging" , "Tired - bored" .)