Size of old canvas shelter (6)
I believe the answer is:
extent
'size' is the definition.
(thesaurus)
'old canvas shelter' is the wordplay.
'old' becomes 'ex'.
'canvas shelter' becomes 'tent' (I've seen this before).
'ex'+'tent'='EXTENT'
'of' acts as a link.
(Other definitions for extent that I've seen before include "stretch" , "Area; size" , "Measure" , "Magnitude - scope" , "Area covered by something" .)