Shed containing old sink (5)
I believe the answer is:
droop
'sink' is the definition.
(to droop is to sink or sag)
'shed containing old' is the wordplay.
'shed' becomes 'drop' (to shed is to cast off or drop).
'containing' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'drop' going around 'o' is 'DROOP'.
(Other definitions for droop that I've seen before include "Hang downwards, lose strength or spirit" , "sink" , "Sag, wilt" , "male problem?" , "languish" .)