A smaller group of alien vessels overhead (6)
I believe the answer is:
subset
'a smaller group' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'alien vessels overhead' is the wordplay.
I cannot quite see how this works, but
'alien' could be 'et' (extra terrestrial) and 'et' is located in the answer.
'vessels' could be 'subs' (sub is a kind of vessel) and 'subs' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for subset that I've seen before include "Not all the members" , "Group within a group" , "Small group nested inside larger group" , "Group of which all the elements are contained in a larger group" , "small class?" .)