Part of Egypt in Asia not entirely developed (5)
I believe the answer is:
sinai
'part of egypt' is the definition.
('Sinai' is part of 'egypt')
'in asia not entirely developed' is the wordplay.
'not entirely' means to remove the last letter (not all of the word).
'developed' indicates anagramming the letters.
'in'+'asia'='inasia'
'inasia' with its final letter taken away is 'inasi'.
'inasi' anagrammed gives 'SINAI'.
(Other definitions for sinai that I've seen before include "11 Across peninsula" , "Egyptian location" , "Mount on which Moses received the Ten Commandments" , "Mount where Moses received God's law" , "Part of 10 [EGYPT]" .)