Particular rampant mushroom in sci-fi production (8)
I believe the answer is:
specific
'particular' is the definition.
('specific' can be similar in meaning to 'particular')
'rampant mushroom in sci-fi production' is the wordplay.
'rampant' is a reversal indicator (rampant can mean rearing).
'mushroom' becomes 'cep' (Boletus edulis - an edible mushroom).
'in' means one lot of letters goes inside another.
'production' is an anagram indicator.
'cep' reversed gives 'pec'.
'sci'+'fi'='scifi'
'scifi' anagrammed gives 'sific'.
'pec' put into 'sific' is 'SPECIFIC'.
(Other definitions for specific that I've seen before include "Explicit or in detail" , "Well-defined" , "Particular, definite" , "Detailed" , "remedy" .)