Head of investigative personnel involved in ace gangster’s trial (12)
I believe the answer is:
experimental
'trial' is the definition.
(synonyms)
'head of investigative personnel involved in ace gangster's' is the wordplay.
'head of' suggests taking the first letters.
'personnel' becomes 'men' (men is a kind of personnel).
'involved' means one lot of letters goes inside another.
'in ace' becomes 'expert' (both can mean a master of something. I am not sure about the 'in' bit.).
'gangster' becomes 'al' (Al Capone).
The initial letter of 'investigative' is 'i'.
'i'+'men'='imen'
'imen' going within 'expert' is 'experiment'.
'experiment'+'al'='EXPERIMENTAL'
(Other definitions for experimental that I've seen before include "Like lab techniques" , "Used as a trial" , "Data-based -- inexpert male (anag)" , "Based on untested ideas" , "Undergoing tests" .)