Do what’s essential to infiltrate old-fashioned right (4,6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

MUST ("what's essential") to infiltrate PASSÉ ("old-fashioned") + R (right) and semi &lit. If the clue is not an &lit., then "what's essential" would be doing double duty as part of the wordplay and part of the solution.