In standard Russian pronunciation based on the Moscow dialect, я or a consonant ending in -я, or one of the soft syllables ча or ща have the vowel reduced to [ɪ] when they occur directly before the stressed syllable.
ча or ща theoretically represent the sound combinations чя and щя, yet since ч and щ are considered inherently soft consonants by their own right, the spelling with -a- is orthographically correct and logical in order to avoid a "double softness" in the spelling.
Note the following:
язы́к [jɪˈzɨk] яйцо́ [jɪjˈʦo] я́йца [ˈjajʦɐ] яи́ц [jɪˈiʦ]
час [tʃas] but часо́в [tʃɪˈsof]
я́сно [ˈjasnɐ] but объясни́ [ɐbjɪsˈɲi]
When in doubt, you can go to this site and enter any word or word form, and it will indicate the pronunciation with all the correct vowel reductions using the cyrillic phonetic transcription, which is easy enough to understand. Just be sure to choose словоформы in the dropdown next to the search field. Otherwise, you'll just get a list of words that rhyme with your entry.