你提供的句子不完整，漏掉了主句，应为：

On the other hand, where this stylistic device is not justified, Baker is also expert in avoiding repetition by employing synonymous words and phrases.

据此，where this stylistic device is not justified 是地点状语从句，不是定语从句，当然就没有先行词了。Baker is also expert in avoiding repetition by employing synonymous words and phrases 是主句。

【补充解答】

where 虽然是引导地点状语从句，但有时候不一定按照地点状语来翻译，而是按照“条件”来理解：“在那方面；在那种情况下；那样”。例如：

There is no peace where men are greedy.

在人们贪婪的情况下是永远不会有安宁的。

On the other hand, where this stylistic device is not justified, Baker is also expert in avoiding repetition by employing synonymous words and phrases.

【翻译】另一方面，在这种文体表现手法并非合理的情况下，贝克擅长使用同义词和同义短语，以避免重复。