Reptile I assume collects dung (9)
I believe the answer is:
iguanodon
'reptile' is the definition.
(iguanodon is a kind of reptile)
'i assume collects dung' is the wordplay.
'assume' becomes 'don' (I've seen this before).
'collects' means one lot of letters goes inside another.
'dung' becomes 'guano' (excrement of sea birds).
'i'+'don'='idon'
'idon' going around 'guano' is 'IGUANODON'.
(Other definitions for iguanodon that I've seen before include "big beast" , "fossil" , "Large dinosaur" , "reptile now extinct" , "In the past a massive herbivore" .)