Mollusc soup cooked by our chef today for starters (7)
I believe the answer is:
octopus
'mollusc' is the definition.
(octopus is a kind of mollusc)
'soup cooked by our chef today for starters' is the wordplay.
'cooked' is an anagram indicator (letters cooked into a new form).
'by' means one lot of letters go next to another.
'for starters' says to take the initial letters.
The initial letters of 'our chef today' is 'oct'.
'soup' anagrammed gives 'opus'.
'opus' put after 'oct' is 'OCTOPUS'.
(Other definitions for octopus that I've seen before include "Eight-armed mollusc" , "Cops out (anag.)" , "Sea creature - pulpo on a Spanish menu" , "one finds, in the main" , "Cephalopod with suckered tentacles" .)