Initial advantage when the master's acrimonious (4,5)
I believe the answer is:
head start
'initial advantage' is the definition.
(I've seen this before)
'the master's acrimonious' is the wordplay.
'the master's' becomes 'heads' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'acrimonious' becomes 'tart' (I've seen this in another clue).
'heads'+'tart'='HEAD START'
'when' is the link.
Can you help me to learn more?
(Other definitions for head start that I've seen before include "Substantial initial advantage" , "advantage, initially" , "Boost?" , "An early advantage for the star ad" , "A good beginning using one's loaf" .)