To give soldiers a break is most unusual! (6)
I believe the answer is:
rarest
'most unusual' is the definition.
(I've seen this before)
'to give soldiers a break' is the wordplay.
'to give' is a charade indicator (letters next to each other).
'soldiers' becomes 'RA' (abbreviation for Royal Artillery).
'a break' becomes 'rest' (a rest is taking a break).
'ra' put next to 'rest' is 'RAREST'.
'is' acts as a link.
(Other definitions for rarest that I've seen before include "Most scarce and probably precious" , "Sartre (anag) -- most uncommon" , "Least common, most unusual" , "Most scarce or uncommon" , "least done" .)