The extent of wind around back of shed (7)
I believe the answer is:
breadth
'the extent' is the definition.
(thesaurus)
'wind around back of shed' is the wordplay.
'wind' becomes 'breath' (breath is a kind of wind**).
'around' is an insertion indicator.
'back of' indicates one should take the final letters.
The last letter of 'shed' is 'd'.
'breath' placed around 'd' is 'BREADTH'.
'of' acts as a link.
(Other definitions for breadth that I've seen before include "Width, thickness" , "Liberality" , "Wide range" , "Wide measurement" , "Extent from side to side" .)