Simpleton caught inside hill (5)
I believe the answer is:
dunce
'simpleton' is the definition.
(dunce is a kind of simpleton)
'caught inside hill' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'inside' means one lot of letters goes inside another.
'hill' becomes 'dune' (I have seen 'Hill of sand ' mean 'dune' so perhaps 'hill' could also mean 'dune').
'c' put into 'dune' is 'DUNCE'.
(Other definitions for dunce that I've seen before include "School fool" , "Term of ridicule applied to followers of John Duns Scotus" , "Bad scholar, may wear a paper cap" , "Poor student" , "One who's bottom of the class?" .)