Dog rolled big rock inside (5)
I believe the answer is:
corgi
'dog' is the definition.
(I know that corgi is a type of dog)
'rolled big rock inside' is the wordplay.
'rolled' says the letters should be written backwards (letters roll back).
'inside' says the answer is hidden in the clue.
'igroc' can be found hidden inside 'big rock'.
'igroc' back-to-front is 'CORGI'.
(Other definitions for corgi that I've seen before include "Dog of Welsh breed, royal favourite" , "Welsh 11" , "Queen likes this" , "HM's favourite dog" , "Welsh breen of dog" .)