What does the cryptic crossword clue "50% reduction on foam rubber that's light in the middle (5)" mean?

The wordplay uses a charade (building blocks). AM (synonym="AM") + BER (synonym="BER") = AMBER The answer is AMBER.

50% reduction on foam rubber that's light in the middle (5)

Guardian Cryptic #26527 | 23 Down | 2015-03-24 flower