Dacoit leader first off, to rob, vanish (7)
I believe the answer is:
dwindle
'vanish' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for dwindle that I've seen before include "Get smaller, less" , "Die away" , "Peter out" , "Degenerate" , "Become gradually less or smaller" .)