Jelly and custard ultimately polished off by youth in outdoor cafe (3,6)
I believe the answer is:
tea garden
'outdoor cafe' is the definition.
(I've seen this before)
'jelly and custard ultimately polished off by youth in' is the wordplay.
'jelly' becomes 'agar' (gelatinous substance).
'and' says to put letters next to each other.
'ultimately' indicates one should take the final letters.
'polished off by youth' becomes 'teen' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
The final letter of 'custard' is 'd'.
'agar'+'d'='agard'
'agard' put into 'teen' is 'TEA-GARDEN'.
Can you help me to learn more?
(Other definitions for tea garden that I've seen before include "Outdoor eatery" , "outdoor restaurant" , "eat alfresco here" , "Kind of plantation" , "Open-air restaurant for afternoon use" .)