Heads of some military units get complacent (4)
I believe the answer is:
smug
'complacent' is the definition.
(thesaurus)
'heads of some military units get' is the wordplay.
'heads of' suggests taking the first letters.
The first letters of 'some military units get' is 'SMUG'.
(Other definitions for smug that I've seen before include "Be envious of" , "Self-satisfied, complacent" , "Self-righteously complacent" , "Arrogantly complacent" , "irritating expression" .)