What does the cryptic crossword clue "In head's absence, did jam for school snack (4)" mean?

The definition is "school snack". The wordplay uses a deletion. TUCK (deletion) = TUCK; definition: "school snack" The answer is TUCK.

In head's absence, did jam for school snack (4)

Times Cryptic #29175 | 20 Across | 2025-03-12 flower