Thin bar in blue (6)
I believe the answer is:
skinny
'thin' is the definition.
(similar in meaning)
'bar in blue' is the wordplay.
'bar' becomes 'inn' (I've seen this in other clues).
'in' is an insertion indicator.
'blue' becomes 'sky' (I've seen this in other clues).
'inn' going into 'sky' is 'SKINNY'.
(Other definitions for skinny that I've seen before include "Spare: coffee with skimmed milk" , "Tight-fitting" , "Of person, very thin" , "stout? Certainly not!" , "Euphemism for a caffe latte with skimmed (non-fat) milk" .)