Mark starts making his opposite (7)
I believe the answer is:
antonym
'his opposite' is the definition.
'antonym' can be an answer for 'opposite' (synonyms). I'm not certain of the 'his' bit.
'mark starts making' is the wordplay.
'mark' becomes 'antony' (I've seen this before).
'starts' indicates taking the first letters.
The first letter of 'making' is 'm'.
'antony'+'m'='ANTONYM'
Can you help me to learn more?
(Other definitions for antonym that I've seen before include "Nick's could be free" , "A word opposite in meaning to another" , "One's import is contrary" , "opposing description" , "Word that means the opposite of another" .)