Beats first couple of thieves with sacks (6)
I believe the answer is:
throbs
'beats' is the definition.
(throb is a kind of beat)
'couple of thieves with sacks' is the wordplay.
'couple of thieves' becomes 'th' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' says to put letters next to each other.
'sacks' becomes 'robs' (I've seen this in another clue).
'th'+'robs'='THROBS'
'first' acts as a link.
Can you help me to learn more?
(Other definitions for throbs that I've seen before include "Beats as a pulse" , "Pulsates, with pain perhaps" , "Beats with persistent rhythm, as heart does" , "Pulsates like heart" , "Beats like the heart" .)