Pester captain boldly going to effect military evacuation (7)
I believe the answer is:
dunkirk
'military evacuation' is the definition.
(Dunkirk evacuation of 1940)
'pester captain boldly going' is the wordplay.
'pester' becomes 'dun' (I've seen this in another clue).
'captain boldly going' becomes 'kirk' (Star Trek's Captain Kirk).
'dun'+'kirk'='DUNKIRK'
'to effect' acts as a link.
(Other definitions for dunkirk that I've seen before include "French port, scene of a 1940 Allied evacuation" , "Channel port" , "French seaport from which British troops were evacuated in 1940" , "Site of nineteen forty nine Allied evacuation" , "Scene of the evacuation of British forces in 1940" .)