Greek god ultimately without end (5)
I believe the answer is:
atlas
'greek god' is the definition.
(Atlas is an example)
'ultimately without end' is the wordplay.
'ultimately' becomes 'atlast' ('at last' can be a synonym of 'ultimately').
'without end' means to remove the last letter.
'atlast' with its final letter taken away is 'ATLAS'.
(Other definitions for atlas that I've seen before include "Reference work" , "The first cervical vertebra" , "Segment of spine" , "Mythological Titan (involved with the world of maps?)" , "Album of maps" .)