Dish in constant use, ultimately (3)
I believe the answer is:
pie
'dish' is the definition.
(I've seen this before)
'constant use ultimately' is the wordplay.
'constant' becomes 'pi' (mathematics: 3.14).
'ultimately' suggests the final letters.
The last letter of 'use' is 'e'.
'pi'+'e'='PIE'
'in' acts as a link.
(Other definitions for pie that I've seen before include "In the sky, it's a hopeless idea" , "Pastry-topped dish" , "Pasty - former Indian coin - mixed type - bird" , "Covered tart" , "Bird - dish - mixed type" .)