Committee chap at party for a military unit (8)
I believe the answer is:
commando
'a military unit' is the definition.
(commando is a kind of military unit)
'committee chap at party' is the wordplay.
'committee' becomes 'com' (abbreviation for committee).
'chap' becomes 'man' (I've seen this before).
'at' means one lot of letters go next to another.
'party' becomes 'do' (do is a kind of party).
'com'+'man'+'do'='COMMANDO'
'for' acts as a link.
(Other definitions for commando that I've seen before include "Green Beret" , "A specially trained soldier" , "Soldier trained for raiding" , "Army unit" , "One soldier" .)