Most important part of US soldier’s time (4)
I believe the answer is:
gist
'most important part' is the definition.
(something's gist is its meaning or significance)
'us soldier's time' is the wordplay.
'us soldier's' becomes 'GI's' (informal term for a US soldier).
'time' becomes 't' (abbreviation).
'gis'+'t'='GIST'
'of' acts as a link.
(Other definitions for gist that I've seen before include "Central idea" , "Main point of the matter" , "Substance or general idea" , "Significance" , "Key point" .)