Crush head of advance party (5)
I believe the answer is:
abash
'crush' is the definition.
(I've seen this before)
'head of advance party' is the wordplay.
'head of' suggests taking the first letters.
'party' becomes 'bash' (term for a party or celebration).
The initial letter of 'advance' is 'a'.
'a'+'bash'='ABASH'
(Other definitions for abash that I've seen before include "Discomfit" , "Humiliate" , "Rattle" , "Strike with embarrassment" , "Cause to feel 4" .)