Filthy neighbourhood, basically a pigpen (5)
I believe the answer is:
nasty
'filthy' is the definition.
(synonyms)
'neighbourhood basically a pigpen' is the wordplay.
'basically' indicates taking the first letters (the first letter could be its root or base).
'pigpen' becomes 'sty' (synonyms).
The initial letter of 'neighbourhood' is 'n'.
'n'+'a'+'sty'='NASTY'
(Other definitions for nasty that I've seen before include "Vile" , "Horrid, repugnant" , "One who sees the future" , "Disgusting" , "Mean" .)