Decided, on reflection, to have it cut up into small pieces (5)
I believe the answer is:
diced
'decided on reflection to have it' is the definition.
I can't judge whether this definition defines the answer.
'cut up into small pieces' is the wordplay.
I cannot really see how this works, but
'cut' could be 'dice' (dicing is a kind of cutting) and 'dice' is located in the answer.
A single letter 'd' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for diced that I've seen before include "Cut into small cubes" , "Risked" , "Cubed" , "Played with small cubes" , "Chopped; gambled" .)