Cold war boast? (4)
I believe the answer is:
crow
'boast?' is the definition.
(crow is a kind of boast)
'cold war' is the wordplay.
'cold' becomes 'c' (eg on taps).
'war' becomes 'row' (I am not sure about this - if you are sure you should believe this answer much more).
'c'+'row'='CROW'
Can you help me to learn more?
(Other definitions for crow that I've seen before include "shout in triumph" , "Common bird" , "Express triumph; bird" , "Blow one's own trumpet" , "One commonly stoned" .)