Washing that's delicate in sack (7)
I believe the answer is:
bathing
'washing' is the definition.
(bathing is a kind of washing)
'delicate in sack' is the wordplay.
'delicate' becomes 'thin' (similar in meaning).
'in' means one lot of letters goes inside another.
'sack' becomes 'bag' (I've seen this in another clue).
'thin' put within 'bag' is 'BATHING'.
'that's' is the link.
(Other definitions for bathing that I've seen before include "Taking a dip" , "Washing by immersion" , "Wallowing" .)