Cause amazement although the gimmick lacks finish (4)
I believe the answer is:
stun
'cause amazement' is the definition.
(I've seen this before)
'although the gimmick lacks finish' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
The remaining letters 'sun' 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 stun that I've seen before include "Surprise greatly or daze" , "Confound" , "Knock cold" , "Daze with nuts" , "Shock, flabbergast" .)