Maybe setter's tight after time off -- it's a mess (4,9)

'a mess' is the definition.
(I've seen this before)

'maybe setter's tight after time off' is the wordplay.
'maybe setter's' becomes 'dog's' (setter is a type of dog).
'tight' becomes 'fast' (both can mean secure).
'after' is a charade indicator (letters next to each other).
'time off' becomes 'break' (eg a break from work).
'fast' put after 'break' is 'breakfast'.
'dogs'+'breakfast'='DOGS BREAKFAST'

'it's' acts as a link.