Leave as, say, saxophone backing comes round second time (6)
I believe the answer is:
desert
'leave' is the definition.
(deserting is a kind of leaving)
'say saxophone backing comes round second time' is the wordplay.
'say saxophone' becomes 'reed' (I am not sure about this - if you are sure you should believe this answer much more).
'backing' shows that the letters should be reversed in order.
'comes round' is an insertion indicator.
'second' becomes 's' (common abbreviation).
'time' becomes 't'.
'reed' back-to-front is 'deer'.
'deer' enclosing 's' is 'deser'.
'deser'+'t'='DESERT'
'as' is the link.
Can you help me to learn more?
(Other definitions for desert that I've seen before include "Abandon; barren waste" , "Abandon - wilderness" , "merit and a > quiet place" , "Wild" , "expanse of land without water" .)