Weight I put on top of cement mixer (5)
I believe the answer is:
tonic
'mixer' is the definition.
(I've seen this before)
'weight i put on top of cement' is the wordplay.
'weight' becomes 'ton' (unit of weight).
'put on' says to put letters next to each other (some letters go on or after others).
'top of' says to take the initial letters.
The initial letter of 'cement' is 'c'.
'ton'+'i'+'c'='TONIC'
(Other definitions for tonic that I've seen before include "Quinine water" , "Stimulant" , "Invigorating drink" , "Fillip" , "Keynote of a scale" .)