Rascal worried by agent, mug? (9)
I believe the answer is:
reprobate
'rascal' is the definition.
(I've seen this before)
'worried by agent mug?' is the wordplay.
'worried' becomes 'ate' (something worrying may eat someone up).
'by' means one lot of letters go next to another.
'agent' becomes 'rep' (short for representative).
'mug?' becomes 'rob' (I've seen this in other clues).
'rep'+'rob'='reprob'
'ate' after 'reprob' is 'REPROBATE'.
(Other definitions for reprobate that I've seen before include "Person without moral scruples" , "base" , "corrupt" , "immoral" , "Villain" .)