Well-ventilated farm building not opening (4)
I believe the answer is:
airy
'well-ventilated' is the definition.
(something well-ventilated has lots of fresh air)
'farm building not opening' is the wordplay.
'farm building' becomes 'dairy'.
'not opening' means to remove the first letter (the opening letter is removed).
'dairy' with its first letter taken away is 'AIRY'.
(Other definitions for airy that I've seen before include "Delicate, casual" , "Flippant" , "Well-ventilated; delicate" , "Delicate; well ventilated" , "open to the sky" .)