Punctuation mark at end of invitation to party for soldier (8)
I believe the answer is:
commando
'for soldier' is the definition.
(I've seen this before)
'punctuation mark at end of invitation to party' is the wordplay.
'punctuation mark' becomes 'comma' (comma is a kind of punctuation mark).
'at end of' indicates one should take the final letters.
'to party' becomes 'do' (do is a kind of party).
The final letter of 'invitation' is 'n'.
'comma'+'n'+'do'='COMMANDO'
(Other definitions for commando that I've seen before include "Member of a special service military unit" , "Specialist soldier" , "shock troops" , "Army unit" , "Green Beret" .)