US measure to besiege terrorist centre? Hang about! (6)
I believe the answer is:
loiter
'hang about' is the definition.
(thesaurus)
'us measure to besiege terrorist centre?' is the wordplay.
'us measure' becomes 'liter' (liter is the US spelling of litre).
'to besiege' means one lot of letters goes inside another.
'centre?' means to look at the middle letters (I've seen 'centre' mean this).
The centre of 'terrorist' is 'o'.
'liter' placed around 'o' is 'LOITER'.
(Other definitions for loiter that I've seen before include "Hang around, lurk" , "Linger aimlessly" , "Linger, tarry" , "Mooch, hang around" , "Wait suspiciously" .)