What does the cryptic crossword clue "Balls up, say for side (4)" mean?

The definition is "side". The wordplay uses a reversal. ED (synonym="ED") + EG (reversal="EG") = EDGE; definition: "side" The answer is EDGE.

Balls up, say for side (4)

Guardian Cryptic #26493 | 22 Down | 2015-02-12 flower