Back yard fencing weak and sagging (6)
I believe the answer is:
droopy
'sagging' is the definition.
('droopy' can be a synonym of 'sagging')
'back yard fencing weak' is the wordplay.
'back' says the letters should be written in reverse.
'yard' becomes 'yd'.
'fencing' means one lot of letters goes inside another (to fence can mean to contain or put a limit round).
'weak' becomes 'poor'.
'yd' enclosing 'poor' is 'ypoord'.
'ypoord' written backwards gives 'DROOPY'.
'and' acts as a link.
(Other definitions for droopy that I've seen before include "Tending to 27 [SAG]" , "Inclined to sag" , "Cartoon dog - sagging" , "Limp" , "Pessimistic, long-eared cartoon dog" .)