Times setter's seen cryptic crossword setter (7)

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

X (times = mathematical symbol for multiplication) + I’M (setter’s = the setter of this crossword is) + anagram (cryptic) of SEEN . Pseudonym used by the cryptic crossword setter Derrick Somerset Macnutt, also a writer on the “rules” of cryptic crosswords.