Greedy au pair with cos in cooking (9)
I believe the answer is:
rapacious
'greedy' is the definition.
(I've seen this before)
'au pair with cos in cooking' is the wordplay.
'with' means one lot of letters go next to another.
'in cooking' indicates anagramming the letters (letters processed or cooked).
'au'+'pair'='aupair'
'aupair' put after 'cos' is 'cosaupair'.
'cosaupair' is an anagram of 'RAPACIOUS'.
(Other definitions for rapacious that I've seen before include "Greedy and grasping" , "Aggressively greedy" , "Extortionate" , "Aggressively predatory" , "wanting more" .)