Starts to time heart rate, observing beating pulse (5)

I believe the answer is:

throb

'pulse' is the definition.
(throb is a kind of pulse)

'starts to time heart rate observing beating' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'time heart rate observing beating' is 'THROB'.

(Other definitions for throb that I've seen before include "Broth (anag.)" , "Pulsate with force" , "Strong regular beat" , "Pulsate strongly" , "Drum" .)

I've seen this clue in The Telegraph.

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