Start to explode (3,3)
I believe the answer is:
set off
I believe this is a double definition.
'start' is the first definition.
(setting off is a kind of starting)
'explode' is the second definition.
('set off' can be a synonym of 'explode')
'to' acts as a link.
(Other definitions for set off that I've seen before include "Begin journey or cause to explode" , "show to advantage" , "Commence journey or detonate" , "Begin journey or cause to ignite" , "Depart" .)