One's unhurried love is put into cake (8)
I believe the answer is:
tortoise
'one's unhurried' is the definition.
(a slow animal)
'love is put into cake' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'put into' is an insertion indicator.
'cake' becomes 'torte' (torte is a kind of cake).
'o'+'is'='ois'
'ois' going inside 'torte' is 'TORTOISE'.
(Other definitions for tortoise that I've seen before include "Slow mover" , "One that kept leading Achilles" , "Would this animal turn to Tories?" , "Slow-moving four-footed reptile - outraced the hare" , "Slow-moving land reptile" .)