Understanding graveyard's first epitaph (4)
I believe the answer is:
grip
'understanding' is the definition.
(I've seen this in another clue)
'graveyard's first epitaph' is the wordplay.
'graveyard's first' becomes 'g' (1st letter of 'graveyard').
'epitaph' becomes 'RIP' (rest in peace).
'g'+'rip'='GRIP'
(Other definitions for grip that I've seen before include "Purchase" , "member of camera crew" , "mastery of problem" , "Stagehand" , "Fascinate" .)