Firstly, I must say that this sounds a bit meaningless to me.The invocation of a news frame is not to be viewed as a means to preclude the encoding of information which might explicit politicize the seemingly impartial definitions of social reality of offer
Secondly, it should be 'excplicitly' and 'on offer'.
I think it really means that news is not objective- it can make poltical staements by embodying assumptions whilst appearing impartial.
However, I could be completely wrong here because I'm really struggling to have any idea of what the author is banging on about. ;-(