What does the cryptic crossword clue "False pretences no sin - but they get the bird (6,8)" mean?

The wordplay uses an anagram. anagram of PRETENCES + NO + SIN = PRISONSENTENCE The answer is PRISONSENTENCE.

False pretences no sin - but they get the bird (6,8)

Guardian Cryptic #22121 | 27 Across | 2001-02-01 flower