Length of garden reaching maturity (7)
I believe the answer is:
yardage
'length' is the definition.
(I've seen this before)
'garden reaching maturity' is the wordplay.
'garden' becomes 'yard' (I have seen 'American garden' mean 'yard' so perhaps 'garden' could also mean 'yard').
'reaching' means one lot of letters go next to another.
'maturity' becomes 'age' (I've seen this before).
'yard'+'age'='YARDAGE'
'of' acts as a link.
(Other definitions for yardage that I've seen before include "Distance gained (Am. football)" , "Distance - day rage (anag)" , "Quantity of fabric" , "use of enclosed space" , "measure" .)