Fuming, missing medium shaped moulds? (5)
I believe the answer is:
fungi
'moulds?' is the definition.
(mould is a kind of fungus)
'fuming missing medium shaped' is the wordplay.
'missing' is a deletion indicator.
'medium' becomes 'm' (as in UK clothing sizes).
'shaped' indicates an anagram.
'fuming' with 'm' removed is 'fuing'.
'fuing' with letters rearranged gives 'FUNGI'.
(Other definitions for fungi that I've seen before include "Mushrooms, toadstools etc (5)" , "chanterelles etc" , "provides penicillin, etc" , "Spore producers" , "life in woodland?" .)