Slow mover, old, is tucking into sweet cake (8)
I believe the answer is:
tortoise
'slow mover' is the definition.
(I've seen this before)
'old is tucking into sweet cake' is the wordplay.
'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).
'o'+'is'='ois'
'ois' inserted within 'torte' is 'TORTOISE'.
(Other definitions for tortoise that I've seen before include "Slow-moving four-footed reptile - outraced the hare" , "This creature might turn to Tories" , "One that kept leading Achilles" , "Would this animal turn to Tories?" , "Shelled reptile" .)