Plant amazing initially, suffering harm when penetrated by insect (8)
I believe the answer is:
amaranth
'plant' is the definition.
(amaranth is a kind of plant)
'amazing initially suffering harm when penetrated by insect' is the wordplay.
'initially' indicates taking the first letters.
'suffering' indicates anagramming the letters.
'when penetrated by' is an insertion indicator.
'insect' becomes 'ant' (ant is a kind of insect**).
The first letter of 'amazing' is 'a'.
'harm' with letters rearranged gives 'marh'.
'marh' going around 'ant' is 'maranth'.
'a'+'maranth'='AMARANTH'
(Other definitions for amaranth that I've seen before include "tropical plant" , "Edible plant; purple" , "South American cereal; purple colour" , "Fabled never-fading flower" , "Never-fading flower; food colour dye" .)