Beat the short mug (5)
I believe the answer is:
throb
'beat' is the definition.
(throb is a kind of beat)
'the short mug' is the wordplay.
'short' means to remove the last letter.
'mug' becomes 'rob' (to mug someone is to rob them).
'the' with its final letter taken away is 'th'.
'th'+'rob'='THROB'
(Other definitions for throb that I've seen before include "Vibrate with rhythm like heart" , "Beat strongly" , "Strong regular beat" , "Strong rhythmic beat" , "Drum" .)