Solutions .. mind (single-topic preoccupation) has 4 entries with similar clues

• 9-letter words
• monomaniaAn obsessive preoccupation with one thing
• obsessionPreoccupation
• 15-letter words
• self-absorptionPreoccupation with one's own emotions
• 18-letter words
• harp-on-one-stringtalk tediously about a single topic