Witness in secret, once withdrawing? (5)
I believe the answer is:
noter
'witness' is the definition.
(I've seen this in another clue)
'in secret once withdrawing?' is the wordplay.
'in' says the answer is hidden in the clue.
'withdrawing?' is a reversal indicator.
'reton' can be found hidden inside 'secret once'.
'reton' back-to-front is 'NOTER'.
(Other definitions for noter that I've seen before include "One observes" , "He jots things down" .)