Molluscs make soft aliens (7)
I believe the answer is:
limpets
'molluscs' is the definition.
(limpet is a kind of mollusc)
'soft aliens' is the wordplay.
'soft' becomes 'limp' (I've seen this in other clues).
'aliens' becomes 'ETs' (ET is short for extraterrestrial).
'limp'+'ets'='LIMPETS'
'make' is the link.
(Other definitions for limpets that I've seen before include "Marine molluscs that cling tightly to rocks" , "Proverbially clingy marine gastropods" , "'Molluscs, they cling to rocks (7)'" .)