Pound starts to tank, heralding run on banks (5)
I believe the answer is:
throb
'pound' is the definition.
(synonyms)
'starts to tank heralding run on banks' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'tank heralding run on banks' is 'THROB'.
(Other definitions for throb that I've seen before include "Broth (anag.)" , "Vibrate with rhythm like heart" , "Sound of engines" , "Pulsate with force" , "Martin -; Kingsley -" .)