Encroaching navvies I suspect (8)
I believe the answer is:
invasive
'encroaching' is the definition.
('invasive' can be a synonym of 'encroaching')
'navvies i suspect' is the wordplay.
'suspect' is an anagram indicator.
'navvies'+'i'='navviesi'
'navviesi' anagrammed gives 'INVASIVE'.
(Other definitions for invasive that I've seen before include "Encroaching" , "Obtrusive, incursive" , "Tending to spread, like plants or disease" , "Tending to intrude" , "Intruding" .)