Reptile coiled oddly around most of boulder (9)
I believe the answer is:
crocodile
'reptile' is the definition.
(I know that crocodile is a type of crocodilian)
'coiled oddly around most of boulder' is the wordplay.
'oddly' indicates anagramming the letters (letters in an odd order).
'around' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'boulder' becomes 'rock' (I've seen this in another clue).
'coiled' anagrammed gives 'codile'.
'rock' with its final letter taken away is 'roc'.
'codile' going around 'roc' is 'CROCODILE'.
(Other definitions for crocodile that I've seen before include "Large reptile with long jaws" , "Recoil, doc, from the reptile" , "Large river reptile" , "Tropical reptile" , "children in pairs?" .)