She stepped back, as if in defence.句法分析

1. as if是连词，除了可以引导一个句子之外，还可以接分词短语/不定式短语或无动词短语

2. as if in defence是状语，方式状语

3. 是后接无动词短语的方式状语

好像防卫似的退了一步。