Mug went back for glue (3)
I believe the answer is:
gum
'glue' is the definition.
(synonyms)
'mug went back' is the wordplay.
'went back' is a reversal indicator.
'mug' back-to-front is 'GUM'.
'for' acts as a link.
(Other definitions for gum that I've seen before include "surrounding roots of teeth?" , "Eg, a eucalyptus" , "Part of mouth; a tree" , "Leaving body" , "Glue; jelly-like sweet" .)