Expression Worn by a Serious Sportsperson
Puzzle 13 of Aquarium Easy Pack
Question is: Expression Worn by a Serious Sportsperson
and the answer is: GAMEFACE
Word Meaning:
GAME FACE — (idiomatic) The expression of one who is prepared for or is facing a lot of difficult and/or undesirable work, especially when it is imminent.
Definitions from Wiktionary.