Mend circumstances broken by tension (6)
I believe the answer is:
stitch
'mend' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'circumstances broken by tension' is the wordplay.
'circumstances' becomes 'sitch' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'broken by' means one lot of letters goes inside another (inserted letters break into the word).
'tension' becomes 't' (abbreviation for surface tension).
'sitch' enclosing 't' is 'STITCH'.
Can you help me to learn more?
(Other definitions for stitch that I've seen before include "in time one could save many" , "Use a needle - pain in the side" , "herringbone?" , "Sharp spasm of pain in the side" , "Loop of thread" .)