Initially believes lapse in progress is temporary setback (4)
I believe the answer is:
blip
'temporary setback' is the definition.
(I've seen this before)
'initially believes lapse in progress' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'believes lapse in progress' is 'BLIP'.
'is' is the link.
(Other definitions for blip that I've seen before include "Small mark - unforeseen deviation" , "Signal on a radar screen" , "Glitch" , "often seen on radar screen" , "Minor and temporary problem" .)