Allied force at once circling German capital (7)
I believe the answer is:
cognate
'allied' is the definition.
(thesaurus)
'force at once circling german capital' is the wordplay.
'force' is an anagram indicator (letters must be forced into a new order).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'capital' indicates taking the first letters.
The first letter of 'german' is 'g'.
'at'+'once'='atonce'
'atonce' is an anagram of 'conate'.
'conate' going around 'g' is 'COGNATE'.
(Other definitions for cognate that I've seen before include "Having the same source or origin" , "Related, having same source" , "Associated" , "Kindred, related" , "Affiliated" .)