What does the cryptic crossword clue "It's one form of refraction, no doubt (3,7)" mean?

The definition is "doubt". The wordplay uses an anagram. anagram of REFRACTION, = FORCERTAIN; definition: "no doubt" The answer is FORCERTAIN.

It's one form of refraction, no doubt (3,7)

Guardian Cryptic #24288 | 5 Down | 2008-01-18 flower