Changing careers with hard study (8)
I believe the answer is:
research
'study' is the definition.
(I know that research is a more specific form of the action investigate)
'changing careers with hard' is the wordplay.
'changing' is an anagram indicator.
'with' says to put letters next to each other.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'careers' anagrammed gives 'researc'.
'researc'+'h'='RESEARCH'
(Other definitions for research that I've seen before include "Thoroughly investigate" , "Systematic investigation to establish facts" , "Scientific investigation" , "Investigate through study" , "Look into" .)