New shelter, essentially, in case it's bright (6)
I believe the answer is:
sunlit
'it's bright' is the definition.
(similar in meaning)
'new shelter essentially in case' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'essentially' means to look at the middle letters.
'in' indicates putting letters inside.
'case' becomes 'suit' (synonyms).
The middle of 'shelter' is 'l'.
'n'+'l'='nl'
'nl' put within 'suit' is 'SUNLIT'.
(Other definitions for sunlit that I've seen before include "enlightened by star" , "Provided with illumination, naturally" , "Bright with solar illumination" , "Not so at night" , "Insult (anag.)" .)