### Clue: Incredibly slender terrains helping mainlands interconnect, primarily? (6)
### Answer: ISTHMI 
 ### Explanation: Incredibly slender terrains helping mainlands interconnect, primarily? (6)
The 'primarily clue: first letters ('primarily') of '**I**ncredibly **S**lender **T**errains **H**elping **M**ainlands **I**nterconnect', &lit.