"is a reference to Noel Coward’s play Blithe Spirit"

This clue uses an anagram - look for an anagram indicator and letters to rearrange

Join the wordplay parts together in sequence

Answer: BLITHE | Wordplay: Coward’s spirit thusly seen in barrel drunk by that man? (6)B (barrel, e.g. of oil) + LIT (drunk) + HE; the definition is a reference to Noel Coward’s play Blithe Spirit