Careers wrecked by hard investigation (8)
I believe the answer is:
research
'investigation' is the definition.
(research is investigating something)
'careers wrecked by hard' is the wordplay.
'wrecked' indicates anagramming the letters.
'by' means one lot of letters go next to another.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'careers' with letters rearranged gives 'researc'.
'researc'+'h'='RESEARCH'
(Other definitions for research that I've seen before include "scientific survey" , "Examination" , "university job" , "Scientific investigation" , "Investigate through study" .)