I ring Enid up for an antiseptic (6)
I believe the answer is:
iodine
'an antiseptic' is the definition.
(iodine is a kind of antiseptic)
'i ring enid up' is the wordplay.
'ring' becomes 'o'.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'enid' written backwards gives 'dine'.
'i'+'o'+'dine'='IODINE'
'for' acts as a link.
(Other definitions for iodine that I've seen before include "Element with symbol I" , "Mother-of-pearl" , "Solution applied to wounds as an antiseptic" , "Infamous tablets in case of a nuclear emergency" , "Element existing in vapour and crystalline form" .)