### Clue: Old boundary miles before lake in Lyme Regis (4)
### Answer: MERE 
 ### Explanation: Definition: Old boundary,lake
 Double wordplay: 1st: M(abbrev. for "miles") + ERE(poetically, before timewise;prior); and 2nd: **Hidden in**(in) "Lyme Regis".
Double defn. too.
Two complete clues in one!