Absence of marines is what one expects (4)
I believe the answer is:
norm
'what one expects' is the definition.
'absence of marines' is the wordplay.
'absence of' becomes 'no'.
'marines' becomes 'rm' (abbreviation for 'Royal Marines').
'no'+'rm'='NORM'
'is' acts as a link.
(Other definitions for norm that I've seen before include "Bacterium, virus, etc" , "Mean" , "Expected standard" , "What is conventional, regular" , "Typical pattern or situation" .)