Slow mover, old, is tucking into sweet cake (8)
I believe the answer is:
Here is my best explanation:
'slow mover' is the definition.
(I've seen this before)
'old is tucking into sweet cake' is the subsidiary indication.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tucking into' means one lot of letters goes inside another.
'sweet cake' becomes 'torte' (I've seen this before).
'ois' put inside 'torte' is 'tortoise'.
(Other definitions for tortoise that I've seen before include "Slow-moving reptile that won race against a hare", "Slow reptile that raced a hare", "Shelled reptile", "One that kept leading Achilles", "One shelled", "he's slow to eat lettuce etc", "Slow-moving four-footed reptile - outraced the hare", "Would this animal turn to Tories?", "one progresses very slowly" and "Reptile with a shell".)