Note some of those turning up (3)
I believe the answer is:
soh
'note' is the definition.
(I've seen this before)
'some of those turning up' is the wordplay.
'some of' indicates the answer is hidden within the clue.
'turning up' shows that the letters should be reversed in order.
'hos' is hidden in the letters of 'those'.
'hos' back-to-front is 'SOH'.
(Other definitions for soh that I've seen before include "5th musical note" , "element of scale" , "It's between fah and la" , "Note; sounds like darn" , "Musical note; sounds like very good" .)