Looking up fungi's description (4)
I believe the answer is:
spec
'description' is the definition.
(spec is a kind of description)
'looking up fungi's' is the wordplay.
'looking up' shows that the letters should be reversed in order.
'fungi's' becomes 'ceps' (name for Boletus edulis - an edible mushroom).
'ceps' written backwards gives 'SPEC'.
(Other definitions for spec that I've seen before include "On - means without a plan but with hope of success" , "Bet" , "Awkward, clumsy" , "Detailed design document (abbr.)" , "estimation?" .)