It follows itch: pulse dropping regularly (4)
I believe the answer is:
thus
'it follows' is the definition.
(both can mean 'therefore')
'itch pulse dropping regularly' is the wordplay.
'dropping regularly' means one should take alternating letters (letters removed at regular intervals).
'itch'+'pulse'='itchpulse'
The alternating letters of 'itchpulse' are 'THUS'.
(Other definitions for thus that I've seen before include "As a result" , "Like so" , "In this manner" , "In this way" , "as indicated" .)