Halfwit caught flies in pile of sand (4)
I believe the answer is:
dune
'pile of sand' is the definition.
(sand dune)
'halfwit caught flies' is the wordplay.
'halfwit' becomes 'dunce' (both can mean a foolish person).
'caught' becomes 'c' (cricket abbreviation).
'flies' suggests deleting specific letters (some letters fly away).
'dunce' with 'c' taken out is 'DUNE'.
'in' acts as a link.
(Other definitions for dune that I've seen before include "Coastal sand hill" , "Sandhill" , "Seashore sand hill" , "Ridge of wind-blown sand" , "Nude in the sand" .)