A bag containing spare washing (7)
I believe the answer is:
bathing
'washing' is the definition.
(bathing is a kind of washing)
'a bag containing spare' is the wordplay.
'containing' is an insertion indicator.
'spare' becomes 'thin' (similar in meaning).
'bag' placed around 'thin' is 'BATHING'.
(Other definitions for bathing that I've seen before include "Wallowing" , "Washing by immersion" , "Taking a dip" .)