Possibly mug popular singer (5)
I believe the answer is:
robin
'singer' is the definition.
(robins are songbirds)
'possibly mug popular' is the wordplay.
'possibly mug' becomes 'rob' (mugging is a type of robbery).
'popular' becomes 'in' ('in' can mean trendy).
'rob'+'in'='ROBIN'
(Other definitions for robin that I've seen before include "Famous outlaw" , "Bird - boy's name" , "Day was one" , "Hood, possibly" , "Bird; Batman's aide" .)