Flower, by the sound of it, that's fastened (4)
I believe the answer is:
tied
'fastened' is the definition.
(tying is a kind of fastening)
'flower by the sound of it' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is located in the answer.
The remaining letters 'id' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for tied that I've seen before include "Made into a knot" , "Made bow" , "Drawn" , "(Of pub) controlled by a brewery" , "limited" .)