### Clue: Replaced X-ray unit, Nigerian, on time (7)
### Answer: REMIBOT 
 ### Explanation: Definition: R
**REM** (an old unit of radiation, so "replaced X-ray unit") + **IBO** ("Nigerian") *on* **T **(time) 
**REMIT*** ("present") around ***BO*** (a "tree" from India and Sri Lanka)