One's joined with soldiers in amendment of that act (10)
I believe the answer is:
attachment
'one's joined' is the definition.
(something joined or attached to something else)
'soldiers in amendment of that act' is the wordplay.
'soldiers' becomes 'men' (men can mean a group of soldiers).
'in' indicates putting letters inside.
'amendment of' indicates an anagram (I've seen this in other clues).
'that'+'act'='thatact'
'thatact' is an anagram of 'attacht'.
'men' going into 'attacht' is 'ATTACHMENT'.
'with' acts as a link.
(Other definitions for attachment that I've seen before include "Document with e-mail" , "Liking" , "Means of securing" , "what might accompany 50 [EMAIL]" , "what often comes with spam?" .)