What does the cryptic crossword clue "Knocked to the left in garret, lamp switch (5)" mean?

The definition is "switch". The wordplay uses hidden reversed. Hidden reversed in "knocked to the left in garRET, LAmp switch" The answer is Alter.

Knocked to the left in garret, lamp switch (5)

Telegraph Prize Cryptic #3047 | 23 Across | 2020-03-15 flower