Hotel in blue, certainly not well-built (6)
I believe the answer is:
skinny
'certainly not well-built' is the definition.
I can't tell whether this defines the answer.
'hotel in blue' is the wordplay.
'hotel' becomes 'inn' (inn is a kind of hotel).
'in' is an insertion indicator.
'blue' becomes 'sky' (sky blue is a shade of blue).
'inn' put into 'sky' is 'SKINNY'.
Can you help me to learn more?
(Other definitions for skinny that I've seen before include "'Scrawny, weedy (6)'" , "Unattractively thin" , "Lacking food" , "Euphemism for a caffe latte with skimmed (non-fat) milk" , "Low-calorie white coffee -- sent any kilt (anag)" .)