Letters in some message mostly taken the wrong way (6)
I believe the answer is:
'letters' is the definition.
(omega is a kind of letter)
'some message mostly taken the wrong way' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'taken the wrong way' is a reversal indicator.
'sagemo' is hidden within 'message mostly'.
'sagemo' written backwards gives 'OMEGAS'.
'in' is the link.
(Other definitions for omegas that I've seen before include "Conclusions" , "letters" , "Greek characters'" , "Foreign characters" .)