A wing-shaped letter! (5)
I believe the answer is:
delta
'letter' is the definition.
(Greek letter)
'a wing-shaped' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'wing' could be 'l' and 'l' is found in the answer.
The remaining letters 'det' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for delta that I've seen before include "Alluvial area - Greek letter" , "Letter - where river divides" , "Alluvial river mouth" , "Extended area at river mouth" , "Foreign character" .)