Clue: Good writer's opening room for another one (5)
Answer: GRIMM
Explanation: Definition: another one
Tricky. A charade of G ('good') plus RIMM, an envelope ('opening') of I'M ('writer's' i.e."the writer is") in RM ('room'). The 'one' being another writer (actually a pair of them , the brothers Jacob and Wilhelm; there are other GRIMMs who might be described as 'writer' solo, but 19A and other clues point to the famous pair).