Help stops every now and then (3)
I believe the answer is:
sos
'help' is the definition.
(emergency abbreviation SOS)
'stops every now and then' is the wordplay.
'every now and then' means one should take alternating letters (letters selected at regular intervals).
The alternate letters of 'stops' are 'SOS'.
(Other definitions for sos that I've seen before include "request for aid" , "Help initially, Mr Morse" , "Help with dots and dashes" , "urgent appeal" , "Distress call" .)