Creature from outer space found on a lump of metal (3,4)
I believe the answer is:
'creature' is the definition.
(sea slug is a kind of creature)
'outer space found on a lump of metal' is the wordplay.
'outer' suggests removing the centre.
'found on' means one lot of letters go next to another.
'lump of metal' becomes 'slug'.
'space' with its middle removed is 'se'.
'from' is the link.
(Other definitions for sea slug that I've seen before include "Creature" , "Shell-less marine mollusc" , "gastropod mollusc" .)