Nookie identified in pursuit of suspicious behaviour in part of southern England (6)
I believe the answer is:
sussex
'part of southern england' is the definition.
(county in the south of England)
'nookie identified in pursuit of suspicious behaviour' is the wordplay.
'nookie' becomes 'sex' (nookie can informally mean sex).
'identified in pursuit of' says to put letters next to each other (some letters go after or pursue others).
'suspicious behaviour' becomes 'sus' (sus can be short for suspicious).
'sex' after 'sus' is 'SUSSEX'.
'in' acts as a link.
(Other definitions for sussex that I've seen before include "One county cricket team ..." , "County on the English channel" , "Southern county, divided into East and West" , "Coastal county" , "South coast county" .)