Last class first to finish (4)
I believe the answer is:
stop
'to finish' is the definition.
(synonyms)
'last class first' is the wordplay.
'last' says to take the final letters.
'first' becomes 'top' (top can mean the best or first).
The final letter of 'class' is 's'.
's'+'top'='STOP'
(Other definitions for stop that I've seen before include "Intercept" , "port of call" , "Halt, whoa, desist" , "Blockage" , "Standstill" .)