Insect invades ordinary weed (8)
I believe the answer is:
plantain
'weed' is the definition.
(I've seen this before)
'insect invades ordinary' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect).
'invades' is an insertion indicator.
'ordinary' becomes 'plain' (I've seen this before).
'ant' placed into 'plain' is 'PLANTAIN'.
(Other definitions for plantain that I've seen before include "Banana-like fruit" , "Tropical plant like a banana" , "Large banana - weed" , "Low-growing plant; type of banana" , "Coarse banana" .)