Class of small alien (3)
I believe the answer is:
set
'class' is the definition.
(I've seen this before)
'small alien' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'alien' becomes 'et' (extra terrestrial).
's'+'et'='SET'
'of' is the link.
(Other definitions for set that I've seen before include "Firm, like jelly" , "Congeal" , "Group of things of the same kind that belong together" , "Badger's burrow" , "Solidify; group" .)