Impassive detective much taken aback (6)
I believe the answer is:
stolid
'impassive' is the definition.
(I know that impassive can be written as stolid)
'detective much taken aback' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'much' becomes 'lots' (I've seen this in another clue).
'taken aback' says the letters should be written in reverse.
'di'+'lots'='dilots'
'dilots' in reverse letter order is 'STOLID'.
(Other definitions for stolid that I've seen before include "cool" , "Bovine" , "Not excitable, unemotional" , "Wooden" , "without much emotion" .)