Fungi with some seaweed one's eating cold (7)

'fungi' is the definition.
(agaric is a kind of fungus)

'some seaweed one's eating cold' is the wordplay.
'some seaweed' becomes 'agar' (I've seen this before).
'one's' becomes 'is' (Roman numeral).
'eating' is an insertion indicator.
'cold' becomes 'c' (eg on taps).
'is' going around 'c' is 'ics'.
'agar'+'ics'='AGARICS'

'with' is the link.