It depends on how good you are at it.
footballer
a footballer
to play football
Footballer
professional footballer
professional footballer
for being a footballer
for being a footballer
Moonlighting is any job one has, secondary to a primary job. It doesn't necessarily relate to the primary job in any way. In most cases the primary job employer is not informed of the employee's secondary job.
subsute football player
famous linguist
Christiano Ronaldo is a footballer by profession.