Clear ocean, thus navy killing time (4,6)
I believe the answer is:
open season
'killing time' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'clear ocean thus navy' is the wordplay.
'clear' becomes 'open' (synonyms).
'ocean' becomes 'sea' (synonyms).
'thus' becomes 'so' (synonyms).
'navy' becomes 'N' (common abbreviation eg in RN Royal Navy).
'open'+'sea'+'so'+'n'='OPEN SEASON'
Can you help me to learn more?
(Other definitions for open season that I've seen before include "When game may be shot" , "when hunting rules are relaxed" , "enjoyed by some sportsmen" , "anyone could be opposition here" , "When game shootable" .)