Contribution is not properly used thus at first (5)
I believe the answer is:
input
'contribution' is the definition.
(I've seen this before)
'is not properly used thus at first' is the wordplay.
'at first' indicates taking the first letters.
The first letters of 'is not properly used thus' is 'INPUT'.
(Other definitions for input that I've seen before include "contributed information" , "Data etc supplied to computer" , "Contribution; enter (data)" , "Data entry" , "store" .)