Clue: Bird mostly sitting on egg, eating round berry (8)
Answer: GOOSEGOG
Explanation: Definition: berry
A charade of GOOS[e] ('bird') minus its last letter ('mostly'; of course, there follows a E in the answer, but that is clued separately) plus EGOG, an envelope ('eating') of O ('round') in 'egg', for the common name for a gooseberry.