And another angel came out of the TEMPLE,
The previous three angels were flying in midheaven (implied in verses 6, 8, and 9). This angel comes out of the TEMPLE.
This creates a spatial problem. Is John seeing a glimpse of the new heaven (Revelation 21:22) where the TEMPLE is the LAMB (Son of Man - previous verse)? Or is it the previous TEMPLE which was opened in heaven (Revelation 11:19)? They could be one and the same.
crying out with a LOUD VOICE to HIM who sat on the cloud,
It appears that this angel is coming directly from Christ and is repeating what he is ordered to announce. This fulfills one of Christ's missions given to Him to perform upon this earth (John 5:17-27-29).
The LOUD VOICE indicates this isn't a localized announcement but rather one to be heard over all the earth.
"Put in your SICKLE and REAP,
This is related to judgment. See Joel 3:9-13-16 and Psalm 75:6-8.
because the HOUR to REAP has come,
The timing, again as in the rest of Revelation, is precise down to the HOUR. This implies the reaping is NOT a prolonged event. Indeed, if it were, some of the harvest would spoil.
Reaping is a judgment process and particularly upon Babylon (Revelation chapters 18 and on). This parallels with prophesy in Jeremiah 51:33 and the parable of the wheat and tares (Matthew 13:24-30) and the explanation (Matthew 13:36-43).
because the HARVEST of the earth is RIPE.
The word "RIPE" here has the connotation of really ripe almost to the point of being overripe. But this is exactly when a harvest should be done and be done quickly.
Christ refers to this in Matthew 7:16-20. With this harvest, the mocking of God will cease (Galatians 6:6-8-10) and the principles of judgment (Romans 2:1-16) will be executed.