Completes puzzle, on reflection admitting surprise (5)
I believe the answer is:
upset
'surprise' is the definition.
(I've seen this before)
'completes puzzle on reflection admitting' is the wordplay.
'admitting' says the answer is hidden in the clue (I've seen 'admits' mean this).
'on reflection' shows that the letters should be reversed in order.
'tespu' is hidden within 'completes puzzle'.
'tespu' backwards is 'UPSET'.
(Other definitions for upset that I've seen before include "Out of sorts" , "Disturb or distress" , "Overturn - worry" , "turned over" , "Disturbed or capsized" .)