Assess time sat working on end of puzzle (8)
I believe the answer is:
estimate
'assess' is the definition.
(I've seen this before)
'time sat working on end of puzzle' is the wordplay.
'working' indicates an anagram.
'on' means one lot of letters go next to another.
'end of' indicates one should take the final letters.
The last letter of 'puzzle' is 'e'.
'time'+'sat'='timesat'
'timesat' with letters rearranged gives 'estimat'.
'estimat'+'e'='ESTIMATE'
(Other definitions for estimate that I've seen before include "Guess the cost" , "Believe" , "Reckon value" , "Calculate in approximate way" , "Make approximate judgement of size or amount" .)