Jumble sale's top stuff bringing little enthusiasm at first (8)
I believe the answer is:
scramble
'jumble' is the definition.
(synonyms)
'sale's top stuff bringing little enthusiasm at first' is the wordplay.
'top' says to take the initial letters.
'stuff' becomes 'cram' (synonyms).
'at first' says to take the initial letters.
The first letter of 'sales' is 's'.
The initial letters of 'bringing little enthusiasm' is 'ble'.
's'+'cram'+'ble'='SCRAMBLE'
(Other definitions for scramble that I've seen before include "keep communication secret" , "Take off hurriedly" , "Motorbike rally" , "Stir during frying, eggs say." , "Climb upwards on motorcyle" .)