One on leave breaks extremely useful item in the tropics (8,3)
I believe the answer is:
mosquito net
'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'leave breaks extremely useful item in the tropics' is the wordplay.
I cannot quite see how this works, but
'leave' could be 'quit' (quitting is a kind of leaving) and 'quit' is found in the answer.
'extremely' could be 'so' ('so' can mean 'extremely' or 'very') and 'so' is present in the remaining letters.
'item' could be 'one' (I've seen this in another clue) and 'one' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
A single letter 'm' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'on' acts as a link.
Can you help me to learn more?
(Other definitions for mosquito net that I've seen before include "Screen used to keep off insects" , "a fine thing to hang" , "Protection against disease" , "protection from insects" , "way to avoid disease" .)