Mostly assist Mark to get position of leadership (4)
I believe the answer is:
helm
'position of leadership' is the definition.
(I've seen this in another clue)
'mostly assist mark' is the wordplay.
'mostly' means to remove the last letter.
'assist' becomes 'help' (synonyms).
'mark' becomes 'm'.
'help' with its last letter taken away is 'hel'.
'hel'+'m'='HELM'
'to get' acts as a link.
(Other definitions for helm that I've seen before include "Steersman's place" , "Steering mechanism" , "Position of leadership" , "Tiller; steer" , "Position of steerer" .)