Pulse of boy after being knocked over (3)
I believe the answer is:
dal
'pulse' is the definition.
(I've seen this before)
'boy after being knocked over' is the wordplay.
'boy' becomes 'lad' (lad is a kind of boy).
'after being knocked over' says the letters should be written in reverse.
'lad' back-to-front is 'DAL'.
'of' is the link.
(Other definitions for dal that I've seen before include "Indian pulse puree" , "Indian dish of pulses" , "Pigeon pea" .)