That's not fair, I like being in bed (6)
I believe the answer is:
biased
'that's not fair' is the definition.
(I've seen this before)
'i like being in bed' is the wordplay.
'like' becomes 'as'.
'being in' means one lot of letters goes inside another.
'i'+'as'='ias'
'ias' put inside 'bed' is 'BIASED'.
(Other definitions for biased that I've seen before include "Having prejudice" , "Having favourite" , "favouring a particular idea" , "Not impartial" , "'One-sided, slanted (6)'" .)