With adult delayed, distributed letter (5)
I believe the answer is:
delta
'with adult delayed distributed letter' is the definition.
'delta' can be an answer for 'letter' (Greek letter). I am not sure about the 'with adult delayed distributed' bit.
'distributed letter' is the wordplay.
'distributed' becomes 'delt' (I can't explain this - if you can you should give a lot more credence to this answer).
'letter' becomes 'a'.
'delt'+'a'='DELTA'
Can you help me to learn more?
(Other definitions for delta that I've seen before include "Letter - where river divides" , "The Greek D (5)" , "Daedalus started with this" , "Wing shape; Greek D" , "Area of alluvial deposit" .)