He is leader in train robbery (5)
I believe the answer is:
heist
'robbery' is the definition.
(I know that heist is a type of robbery)
'he is leader in train' is the wordplay.
'leader in' says to take the initial letters.
The initial letter of 'train' is 't'.
'he'+'is'+'t'='HEIST'
(Other definitions for heist that I've seen before include "Mugging" , "crime?" , "bank job in America?" , "Armed hold-up (sl.)" , "Armed robbery" .)