Extent of wind across Germany (7)
I believe the answer is:
breadth
'extent' is the definition.
(thesaurus)
'wind across germany' is the wordplay.
'wind' becomes 'breath' (breath is a kind of wind**).
'across' is an insertion indicator.
'germany' becomes 'd' (international vehicle registration code).
'breath' going around 'd' is 'BREADTH'.
'of' acts as a link.
(Other definitions for breadth that I've seen before include "Extent from side to side" , "Wide measurement" , "Wideness" , "Lateral extent" , "Red bath (anag)" .)