Dinosaur dung spreading, I assume (9)
I believe the answer is:
iguanodon
'dinosaur' is the definition.
(I know that iguanodon is a type of dinosaur)
'dung spreading i assume' is the wordplay.
'dung' becomes 'guano' (excrement of sea birds).
'spreading' means one lot of letters goes inside another (inserted letters spread others open).
'assume' becomes 'don' (I've seen this in another clue).
'i'+'don'='idon'
'guano' placed within 'idon' is 'IGUANODON'.
(Other definitions for iguanodon that I've seen before include "It used to eat plants" , "Massive dinosaur of the early Cretaceous period" , "In the past a massive herbivore" , "Herbivorous dinosaur" , "reptile now extinct" .)