Open with nothing to replace a pin (6)
I believe the answer is:
brooch
'a pin' is the definition.
(brooch is a kind of pin)
'open with nothing to replace' is the wordplay.
I cannot really understand how this works, but
'nothing' could be 'o' (looks like zero - 0) and 'o' is located in the answer.
The remaining letters 'broch' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for brooch that I've seen before include "Jewellery item with pin" , "Decorative pin" , "Decorative clasp" , "Ornamental hinged pin with clasp" , "Ornament worn fastened to clothing" .)