Clue: Heading for rehabilitation, is one held within Pollsmoor's walls? (8)
Answer: PRISONER
Explanation: Heading for rehabilitation, is one held within Pollsmoor's walls? (8)
An envelope ('held within') of R ('heading for **R**ehabilitation') plus 'is one' in PR ('**P**ollsmoo**R**'s walls'), with an &lit definition. Pollsmoor is a South African prison.