Farm building lacking roof is well ventilated (4)
I believe the answer is:
airy
'well ventilated' is the definition.
(I've seen this before)
'farm building lacking roof' is the wordplay.
'farm building' becomes 'dairy'.
'lacking roof' means to remove the first letter.
'dairy' with its first letter taken off is 'AIRY'.
'is' is the link.
(Other definitions for airy that I've seen before include "Light and insubstantial" , "Spacious - aerial" , "Breezy" , "Well-ventilated; delicate" , "Old astronomer" .)