Military command: go after crippled U-boat (5-4)
I believe the answer is:
about-turn
'military command' is the definition.
(I've seen this before)
'go after crippled u-boat' is the wordplay.
'go' becomes 'turn' ('turn' can be a synonym of 'go'**).
'after' means one lot of letters go next to another.
'crippled' is an anagram indicator.
'u'+'boat'='uboat'
'uboat' with letters rearranged gives 'about'.
'turn' put after 'about' is 'ABOUT-TURN'.
(Other definitions for about-turn that I've seen before include "Total change of direction" , "Change of opinion" , "Act of pivoting 180 degrees" .)