Top of petunia eaten by land reptile (8)
I believe the answer is:
terrapin
'reptile' is the definition.
(terrapin is a kind of reptile)
'top of petunia eaten by land' is the wordplay.
'top of' indicates taking the first letters.
'eaten by' is an insertion indicator.
'land' becomes 'terrain' (I've seen this before**).
The first letter of 'petunia' is 'p'.
'p' inserted within 'terrain' is 'TERRAPIN'.
(Other definitions for terrapin that I've seen before include "Small freshwater turtle" , "Little turtle" , "Web-footed turtle" , "temporary building" , "Swimmer" .)