Problem about hot space for edible fungus (8)
I believe the answer is:
mushroom
'edible fungus' is the definition.
(I've seen this before)
'problem about hot space' is the wordplay.
'problem' becomes 'sum' (arithmetic problem**).
'about' shows that the letters should be reversed in order.
'hot' becomes 'h'.
'space' becomes 'room' (room is a kind of space).
'sum' back-to-front is 'mus'.
'mus'+'h'+'room'='MUSHROOM'
'for' is the link.
(Other definitions for mushroom that I've seen before include "Pinkish-brown colour" , "Rapidly increase; fungus" , "food in the field" , "expand rapidly" , "Fleshy fungus" .)