Push in; his leader is in charge (6)
I believe the answer is:
thrust
'push' is the definition.
(I know that push can be written as thrust)
'his leader is in charge' is the wordplay.
'leader' suggests taking the first letters.
'is in' indicates putting letters inside.
'charge' becomes 'trust' (I can't justify this - if you can you should give a lot more credence to this answer).
The initial letter of 'his' is 'h'.
'h' placed within 'trust' is 'THRUST'.
'in' is the link.
Can you help me to learn more?
(Other definitions for thrust that I've seen before include "Push suddenly and violently" , "force in argument" , "Push forward, as fencer does sword" , "Pierce" , "Driving force" .)