Beat the short boy (5)
I believe the answer is:
throb
'beat' is the definition.
(throb is a kind of beat)
'the short boy' is the wordplay.
'short' means to remove the last letter.
'boy' becomes 'rob' (**).
'the' with its last letter removed is 'th'.
'th'+'rob'='THROB'
(Other definitions for throb that I've seen before include "Beat rhythmically" , "Pound" , "Strong rhythmic beat" , "Drum" , "beat repeatedly" .)