Supports strikebreaker over accepting sack at last (5)
I believe the answer is:
backs
'supports' is the definition.
(back is a kind of support)
'strikebreaker over accepting sack at last' is the wordplay.
'strikebreaker' becomes 'scab' ('scab' can be a synonym of 'strikebreaker').
'over' says the letters should be written backwards.
'accepting' indicates putting letters inside.
'at last' suggests the final letters.
The last letter of 'sack' is 'k'.
'scab' in reverse letter order is 'bacs'.
'bacs' going around 'k' is 'BACKS'.
(Other definitions for backs that I've seen before include "Gives support or one's blessing to" , "delightful Cambridge area" , "Helps or supports with money" , "Defenders" , "Supports or puts bet on" .)