Son entering wasteland — a fool? (7)
I believe the answer is:
dessert
'a fool?' is the definition.
(fool is a type of dessert)
'son entering wasteland' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'entering' means one lot of letters goes inside another.
'wasteland' becomes 'desert' (I've seen this before).
's' going into 'desert' is 'DESSERT'.
(Other definitions for dessert that I've seen before include "Sweet, last course of a meal" , "Sweet, afters" , "Spotted Dick is one" , "End of meal" , "Pudding, sweet" .)