Carers he involved in investigation (8)
I believe the answer is:
research
'investigation' is the definition.
(I know that research is a type of investigation)
'carers he involved' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'carers'+'he'='carershe'
'carershe' is an anagram of 'RESEARCH'.
'in' acts as a link.
(Other definitions for research that I've seen before include "material for investigation" , "university job" , "Experimentation" , "Fact-finding" , "Investigate through study" .)