She's left a mug (4)
I believe the answer is:
lass
'she's' is the definition.
(I've seen this before)
'left a mug' is the wordplay.
'left' becomes 'l' (common abbreviation).
'a mug' becomes 'ass' (I've seen this before).
'l'+'ass'='LASS'
(Other definitions for lass that I've seen before include "Familiar form of address for female" , "Sweet resident of Richmond Hill?" , "Sweetheart" , "Scottish girl" , "Damsel" .)