Warship went down, part of larger group (6)
I believe the answer is:
subset
'part of larger group' is the definition.
(a subset is part of a larger set)
'warship went down' is the wordplay.
'warship' becomes 'sub' (short for submarine, a submersible warship).
'went down' becomes 'set' (eg the sun goes down or sets).
'sub'+'set'='SUBSET'
(Other definitions for subset that I've seen before include "A collection of things forming part of a larger collection" , "Group of which all the elements are contained in a larger group" , "Not all the members" , "small class?" , "Small group nested inside larger group" .)