Clue: Centrally imposed overtime, holder of innumerable letters — one's job (6)
Answer: POSTIE
Explanation: Definition: letters — one's job
Central letters imPOSed + T (taking "overtime" as "over time") + the outer letters or "holder" of I[nnumerabl]E ~~, with an &littish definition~~ *Thanks to Lord Jim for clarifying the definition