A little secret always turns up in due course (5)
I believe the answer is:
later
'in due course' is the definition.
(I've seen this before)
'a little secret always turns up' is the wordplay.
'a little' says the answer is hidden in the clue (a little bit of the clue).
'turns up' says the letters should be written in reverse.
'retal' is hidden within 'secret always'.
'retal' written backwards gives 'LATER'.
(Other definitions for later that I've seen before include "Alter (anag.)" , "behind schedule" , "After a given point in time" , "At a time in the future" , "after the rest?" .)