Dream offered by a chain letter is dodgy (6,2,3,3)
I believe the answer is:
castle in the air
'dream' is the definition.
(castle in the air is a kind of dream)
'a chain letter is dodgy' is the wordplay.
'dodgy' is an anagram indicator (a dodgy or wrong order of the letters).
'a'+'chain'+'letter'+'is'='achainletteris'
'achainletteris' anagrammed gives 'CASTLE IN THE AIR'.
'offered by' acts as a link.
(Other definitions for castle in the air that I've seen before include "Daydreaming" , "A visionary project" , "Keep up!" , "like the superfortress was?" .)