He's a spy in Barking that's both tough and useless (9)
I believe the answer is:
sisyphean
'both tough and useless' is the definition.
(a Sisyphean task is difficult and futile)
'he's a spy in barking' is the wordplay.
'barking' indicates anagramming the letters (UK slang for mad).
'hes'+'a'+'spy'+'in'='hesaspyin'
'hesaspyin' with letters rearranged gives 'SISYPHEAN'.
'that's' acts as a link.
(Other definitions for sisyphean that I've seen before include "Denoting an unfinishable task" , "requiring unending effort" , "Endlessly laborious" , "uphill" , "Like tragic character's work" .)