What does the cryptic crossword clue "Beam and grate left unfinished (4)" mean?

The definition is "Beam". The wordplay uses a deletion. GRIN (deletion) = GRIN; definition: "Beam" The answer is GRIN.

Beam and grate left unfinished (4)

Guardian Cryptic #28995 | 15 Across | 2023-02-16 flower