Stuff rug with preparation of real insecticide at first (8)
I believe the answer is:
material
'stuff' is the definition.
(synonyms)
'rug with preparation of real insecticide at first' is the wordplay.
'rug' becomes 'mat' (I've seen this before).
'with' means one lot of letters go next to another.
'preparation of' is an anagram indicator (I've seen this in other clues).
'at first' indicates taking the first letters.
The first letter of 'insecticide' is 'i'.
'real'+'i'='reali'
'reali' is an anagram of 'erial'.
'mat'+'erial'='MATERIAL'
(Other definitions for material that I've seen before include "Substantial -- textile" , "Fabric, stuff" , "Fabric or other substance" , "of serious importance" , "Relevant - fabric" .)