Perhaps Mario Balotelli's header, excellent at first, goes over the bar ultimately (7)
I believe the answer is:
plumber
'perhaps mario' is the definition.
I can't judge whether this defines the answer.
'balotelli's header excellent at first goes over the bar ultimately' is the wordplay.
'header' says to take the initial letters (in a down clue, the letter at the top of the word).
'excellent' becomes 'plum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'at first' means one lot of letters go next to another.
'goes over' says to put letters next to each other.
'ultimately' suggests the final letters.
The first letter of 'balotelli' is 'b'.
The final letters of 'the bar' are 'er'.
'b' after 'plum' is 'plumb'.
'plumb'+'er'='PLUMBER'
Can you help me to learn more?
(Other definitions for plumber that I've seen before include "One fits water and gas pipes" , "He fixes water-pipes etc in building" , "One instals and repairs water and gas fittings" , "tradesperson" , "tank specialist?" .)