What does the cryptic crossword clue "Partly fills in useless hole in the head (5)" mean?

The definition is "head". The wordplay uses a hidden word. Hidden in "partly fillS IN USeless hole in the head" The answer is SINUS.

Partly fills in useless hole in the head (5)

Guardian Cryptic #28232 | 23 Down | 2020-09-07 flower