Mysterious utterances that could become so clear (7)
I believe the answer is:
oracles
'mysterious utterances' is the definition.
(I've seen this before)
'could become so clear' is the wordplay.
'could become' is an anagram indicator.
'so'+'clear'='soclear'
'soclear' with letters rearranged gives 'ORACLES'.
'that' acts as a link.
(Other definitions for oracles that I've seen before include "Authoritative statements" , "mysterious utterances" , "Prophets, seers" , "They would be asked questions" , "Prophetic authorities" .)