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' means one lot of letters goes inside another.
'deserted' becomes 'd'.
'mountains' becomes 'range' (range of mountains).
The initial letter of 'expeditions' is 'e'.
'd'+'range'='drange'
'e' going into '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" .)