问心无愧是一个成语。

"Having a clear conscience" is an idiom.

Source: Tatoeba

Sentence Composition

1 问心无愧 wènxīnwúkuì lit. look into one's heart, no shame (idiom); with a clear conscience
=2 是 shì
  1. to be (followed by substantives only)
  2. correct; right; true
  3. (respectful acknowledgement of a command) very well
  4. (adverb for emphatic assertion)
=2 是 shì variant of 是 shì
=3 一个 yīge
  1. a; an; one
  2. the whole (afternoon, summer vacation etc)
=3 一个 yīgè
  1. Combination of the numeral 一 (“one”) and the generic classifier 個／个: a; an; one
  2. same
4 成语 chéngyǔ Chinese set expression, typically of 4 characters, often alluding to a story or historical quotation; idiom; proverb; saying; adage

classifiers: 條, 本, 句