Complete tons of tests assessing liabilities at first (5)
I believe the answer is:
total
'complete' is the definition.
('total' can be a synonym of 'complete')
'tons of tests assessing liabilities at first' is the wordplay.
'tons' becomes 't'.
'at first' says to take the initial letters.
The initial letters of 'of tests assessing liabilities' is 'otal'.
't'+'otal'='TOTAL'
(Other definitions for total that I've seen before include "energy provider" , "Complete, entire amount" , "Complete; add up" , "Unqualified" , "Come to" .)