Almost all of group in on mischievous study (8)
I believe the answer is:
research
'study' is the definition.
(I know that research is a more specific form of investigate)
'almost all of group in on mischievous' is the wordplay.
'almost all of' means to remove the last letter.
'group' becomes 'set' (both can mean a collection).
'in' indicates putting letters inside.
'on' becomes 're' (regarding).
'mischievous' becomes 'arch' (arch can mean mischievous or playful).
'set' with its last letter taken away is 'se'.
're'+'arch'='rearch'
'se' placed into 'rearch' is 'RESEARCH'.
(Other definitions for research that I've seen before include "Experimentation" , "Fact-finding" , "Look into" , "Scientific study" , "Examination" .)