Aloof detectives thrash head of thieves (7)
I believe the answer is:
distant
'aloof' is the definition.
(I've seen this before)
'detectives thrash head of thieves' is the wordplay.
'detectives' becomes 'dis' (Detective Inspectors, DIs).
'thrash' becomes 'tan' (I've seen this before).
'head of' says to take the initial letters.
The first letter of 'thieves' is 't'.
'dis'+'tan'+'t'='DISTANT'
(Other definitions for distant that I've seen before include "Far from - reserved" , "Absent - coming from a far-away place" , "Remote in time or place" , "Far away" , "cool" .)