Mark employed in developing master plan (9)
I believe the answer is:
stratagem
'plan' is the definition.
(stratagem is a kind of plan)
'mark employed in developing master' is the wordplay.
'mark' becomes 'tag' (tag is a kind of mark**).
'employed in' is an insertion indicator.
'developing' indicates anagramming the letters.
'master' is an anagram of 'straem'.
'tag' going within 'straem' is 'STRATAGEM'.
(Other definitions for stratagem that I've seen before include "Tactic to gain an advantage" , "Scheme to outwit an opponent" , "Clever scheme" , "Start game with cunning plan" , "A cunning plan or piece of trickery" .)