Ace marksman has stolen ahead (7)
I believe the answer is:
hotshot
'ace' is the definition.
(I know that ace can be written as hotshot)
'marksman has stolen ahead' is the wordplay.
'marksman' becomes 'shot' (I've seen this in another clue).
'has' means one lot of letters go next to another.
'stolen ahead' becomes 'hot' (stolen goods are 'hot'. I am not sure about the 'ahead' bit.).
'shot' put after 'hot' is 'HOTSHOT'.
Can you help me to learn more?
(Other definitions for hotshot that I've seen before include "Top player" , "Showily successful person" , "Expert" , "ace" , "Important person" .)