Account for what was once obvious
I believe the answer is:
explain
'account for' is the definition.
(I've seen this before)
'once obvious' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'obvious' becomes 'plain' (similar in meaning).
'ex'+'plain'='EXPLAIN'
'what was' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for explain that I've seen before include "Interpret" , "Spell out" , "Show the whys and wherefores" , "Give reasons for" , "Make comprehensible" .)