Pointed mention of pie in the sky (5)
I believe the answer is:
spiky
'pointed' is the definition.
(thesaurus)
'mention of pie in the sky' is the wordplay.
'mention of pie' becomes 'pi' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'pi' going within 'sky' is 'SPIKY'.
Can you help me to learn more?
(Other definitions for spiky that I've seen before include "Having sharp prongs" , "having ears" .)