Get rid of piles and onset of diarrhoea with treatment (6)
I believe the answer is:
dispel
'get rid of' is the definition.
(I've seen this in another clue)
'piles and onset of diarrhoea with treatment' is the wordplay.
'and' is a charade indicator (letters next to each other).
'onset of' indicates taking the first letters.
'with treatment' is an anagram indicator (letters treated or processed).
The first letter of 'diarrhoea' is 'd'.
'piles' after 'd' is 'dpiles'.
'dpiles' is an anagram of 'DISPEL'.
(Other definitions for dispel that I've seen before include "Banish (from the mind)" , "Clear, blow, away" , "Blow away, scatter" , "'Disperse, clear away (6)'" , "Lisped (anag)" .)