One who’s paid in military emergency requiring cool heads (4)
I believe the answer is:
merc
'one who's paid' is the definition.
I can't judge whether this definition defines the answer.
'military emergency requiring cool heads' is the wordplay.
'heads' suggests taking the first letters.
The initial letters of 'military emergency requiring cool' is 'MERC'.
'in' is the link.
Can you help me to learn more?
(Other definitions for merc that I've seen before include "Former cop" , "business-class car" , "High-performance German car" , "German car (abbr)" , "German make of luxury car (colloq.)" .)