Piled in heap, dogs off lead showed visible distress (7)
'piled in heap' is the definition.
(swept or piled up)
'dogs off lead showed visible distress' is the wordplay.
'dogs' becomes 'pups'.
'off lead' means to remove the first letter (the letter which leads the word is taken off).
'showed visible distress' becomes 'wept' (to weep is to show distress).
'pups' with its initial letter removed is 'ups'.
(Other definitions for upswept that I've seen before include "Affected by hair-raising activity", "As Pompadour was", "(Of hair) brushed from front to back", "Shaped as a rising curve".)