Sick conifers get such investigation (8)
I believe the answer is:
forensic
'such investigation' is the definition.
(as in forensic investigation)
'sick conifers' is the wordplay.
'sick' indicates an anagram.
'conifers' is an anagram of 'FORENSIC'.
'get' acts as a link.
(Other definitions for forensic that I've seen before include "Branch of medicine dealing with the law" , "Related to law courts" , "connected with courts of law" , "Using science to investigate crime" , "As scientific tests in crime fighting" .)