Tacky individual originally in very small plainclothes unit (6)
I believe the answer is:
viscid
'tacky' is the definition.
(thesaurus)
'individual originally in very small plainclothes unit' is the wordplay.
'originally' suggests taking the first letters.
'in' is an insertion indicator.
'very' becomes 'v' (abbreviation).
'small' becomes 's' (abbreviation - e.g. clothes size).
'plainclothes unit' becomes 'cid' (criminal investigation department).
The first letter of 'individual' is 'i'.
'v'+'s'='vs'
'i' going into 'vs' is 'vis'.
'vis'+'cid'='VISCID'
(Other definitions for viscid that I've seen before include "Cohesive and sticky" , "Sticky, gluey" , "Of liquid, thick and gluey" , "Of a consistency like treacle" , "Glutinous, sticky" .)