Arrive at bar at last, supporting tipsy reveller (10)
I believe the answer is:
merrymaker
'reveller' is the definition.
(synonyms)
'arrive at bar at last supporting tipsy' is the wordplay.
'arrive at' becomes 'make' (synonyms).
'at last' suggests the final letters.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'tipsy' becomes 'merry' (I've seen this before).
The final letter of 'bar' is 'r'.
'make'+'r'='maker'
'maker' after 'merry' is 'MERRYMAKER'.
(Other definitions for merrymaker that I've seen before include "One having fun" , "Reveller" .)