Celebrity given sack, so show off (4-4)
I believe the answer is:
name-drop
'show off' is the definition.
(I've seen this before)
'celebrity given sack' is the wordplay.
'celebrity' becomes 'name' (as in a 'big name').
'given' says to put letters next to each other.
'sack' becomes 'drop' (I've seen this before).
'name'+'drop'='NAME-DROP'
'so' acts as a link.
(Other definitions for name-drop that I've seen before include "Refer to famous people in familiar way to impress" , "Casually mention famous people you know" , "Boast of acquaintance" , "Casually mention VIP friends" , "Mention celebrities to impress" .)