Keeps omitting to include the salts (5)
I believe the answer is:
epsom
'the salts' is the definition.
(I've seen this before)
'keeps omitting to include' is the wordplay.
'to include' says the answer is hidden in the clue.
'EPSOM' can be found hidden inside 'keeps omitting'.
(Other definitions for epsom that I've seen before include "Surrey racecourse" , "English racecourse" , "Poems about English Derby venue" , "Poems about English racing venue" , "can have horses" .)