Regulars in Boer War imprisoning cold killer (4)
I believe the answer is:
orca
'killer' is the definition.
('orca' can be a synonym of 'killer')
'regulars in boer war imprisoning cold' is the wordplay.
'regulars in' means one should take alternating letters.
'imprisoning' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'boer'+'war'='boerwar'
The alternate letters of 'boerwar' are 'ora'.
'ora' enclosing 'c' is 'ORCA'.
(Other definitions for orca that I've seen before include "Killer-whale" , "grampus" , "School killer" , "Danger for swimmers" , "Large mammal" .)