Unemotional when an officer comes to a sad end (4)
I believe the answer is:
cold
'unemotional' is the definition.
(I've seen this in another clue)
'an officer comes to a sad end' is the wordplay.
'an officer' becomes 'col' (short for colonel).
'comes to' says to put letters next to each other.
'end' suggests the final letters.
The last letter of 'sad' is 'd'.
'col'+'d'='COLD'
'when' acts as a link.
(Other definitions for cold that I've seen before include "Frigid" , "At low temperature" , "Stand-offish" , "Lacking affection" , "Lacking emotion" .)