Conscription: several in action at sea (8,7)
I believe the answer is:
national service
'conscription' is the definition.
(I've seen this before)
'several in action at sea' is the wordplay.
'at sea' indicates an anagram.
'several'+'in'+'action'='severalinaction'
'severalinaction' is an anagram of 'NATIONAL SERVICE'.
(Other definitions for national service that I've seen before include "In a period with the military" , "What conscripts had to do" , "Period of compulsory conscription" , "Period of peacetime conscription" , "Compulsory period in the forces" .)