Engineer unclear initially how this provides propulsion (8)
I believe the answer is:
launcher
'this provides propulsion' is the definition.
(as in a rocket launcher)
'engineer unclear initially how' is the wordplay.
'engineer' is an anagram indicator.
'initially' suggests taking the first letters.
The first letter of 'how' is 'h'.
'unclear'+'h'='unclearh'
'unclearh' is an anagram of 'LAUNCHER'.
(Other definitions for launcher that I've seen before include "Missile firer" , "Rocket firer" , "starter" .)