Send crazy expedition's leader into deserted mountains (7)
I believe the answer is:
derange
'send crazy' is the definition.
I can't judge whether this defines the answer.
'expedition's leader into deserted mountains' is the wordplay.
'leader' indicates taking the first letters.
'into' is an insertion indicator.
'deserted' becomes 'd'.
'mountains' becomes 'range' (range of mountains).
The first letter of 'expeditions' is 'e'.
'd'+'range'='drange'
'e' going within 'drange' is 'DERANGE'.
Can you help me to learn more?
(Other definitions for derange that I've seen before include "Put into confusion" , "Throw into disorder" , "Craze" , "Remove from order" , "Unhinge" .)