Length of garden season (7)
I believe the answer is:
yardage
'length' is the definition.
(I've seen this before)
'garden season' is the wordplay.
'garden' becomes 'yard' (I have seen 'American garden' mean 'yard' so perhaps 'garden' could also mean 'yard').
'season' becomes 'age' (season can mean a period or age).
'yard'+'age'='YARDAGE'
'of' is the link.
(Other definitions for yardage that I've seen before include "Distance measured in imperial units" , "cost of storing" , "use of enclosed space" , "Distance sometimes measured in paces" , "Amount of length (imperial)" .)